Methods for assaying gene imprinting and methylated cpg islands

ABSTRACT

Genomic imprinting is a parent of origin-dependent gene silencing that involves marking of alleles in the germline and differential expression in somatic cells of the offspring. Imprinted genes and abnormal imprinting have been implicated in development, human disease, and embryonic stem cell transplantation. We have established a model system for genomic imprinting using pluripotent 8.5 d.p.c. mouse embryonic germ (EG) cell lines derived from an interspecific cross. We find that allele-specific imprinted gene expression has been lost in these cells. However, partial restoration of allele-specific silencing can occur for some imprinted genes after in vitro differentiation of EG cells into somatic cell lineages, indicating the presence of a gametic memory that is separable from allele-specific gene silencing. We have also generated a library containing most methylated CpG islands. A subset of these clones was analyzed and revealed a subdivision of methylated CpG islands into 4 distinct subtypes: CpG islands belonging to high copy number repeat families; unique CpG islands methylated in all tissues; unique methylated CpG islands that are unmethylated in the paternal germline; and unique CpG islands methylated in tumors. This approach identifies a methylome of methylated CpG islands throughout the genome.

This application claims the benefit of application Ser. Nos. 60/206,158and 60/206,161 filed May 22, 2000.

This invention was made using funds from the U.S. government under agrant from the National Institutes of Health numbered CA65145. The U.S.government therefore retains certain rights in the invention.

BACKGROUND OF THE INVENTION

Genomic imprinting is a parental origin-specific gene silencing thatleads to differential expression of the two alleles of a gene inmammalian cells. Imprinting has attracted intense interest for severalreasons: (i) Imprinting is by definition reversible and may be regulatedover a large genomic domain (1). (ii) Imprinted genes and the imprintingmechanism itself are important in human birth defects and cancer (2).(iii) It has been suggested that imprinting cannot be reprogrammedwithout passage through the germline and thus constitutes a barrier tohuman embryonic stem cell transplantation (3).

Experimental studies of the timing and mechanism of genomic imprintinghave been hampered by the fact that imprinting requires passage throughthe germline, analysis of which poses a difficult experimental targetThus, there is a need in the art for an experimental model system whichallows direct examination of allele-specific gene silencing in thedynamic process of genomic imprinting.

DNA methylation is central to many mammalian processes includingembryonal development, X-inactivation, genomic imprinting, regulation ofgene expression, and host defense against parasites, as well as abnormalprocesses such as carcinogenesis, fragile site expression, and cytosineto thymine transition mutations. DNA methylation in mammals is achievedby the transfer of a methyl group from S-adenosyl-methionine to the C5position of cytosine. This reaction is catalyzed by DNAmethyltransferases and is specific to cytosines in CpG dinucleotides.70% of all cytosines in CpG dinucleotides in the human genome aremethylated and prone to deamination, resulting in a cytosine to thyminetransition. This process leads to an overall reduction in the frequencyof guanine and cytosine to about 40% of all nucleotides and a furtherreduction in the frequency of CpG dinucleotides to about a quarter oftheir expected frequency (35). The exception to this rule are CpGislands, that were first identified as HpaII tiny fragments (36), laterto be defined as sequences of 1-2 kb with a GC content of above 50% anda frequency of CpG dinucleotides greater than 0.6 of their expectedfrequency (37). CpG islands have been estimated to constitute 1-2% ofthe mammalian genome (38), and are found around the promoters of allhousekeeping genes, as well as in a less conserved position in 40% oftissue specific genes (39). The persistence of CpG dinucleotides in CpGislands is largely attributed to a general lack of methylation,regardless of expression status (reviewed in ref. 40).

The two exceptions to the rule of CpG islands being unmethylated innormal cells, are on the inactive X chromosome (41) and in associationwith imprinted genes (42,43). Genomic imprinting is the differentialexpression of the two parental alleles of a gene, and most imprintedgenes are associated with at least one CpG island methylated uniquely ona specific parental chromosome (42). In addition, aberrant methylationof CpG islands has been observed in tumors and cultured cells, and it isthought to be a mechanism to silence tumor suppressor genes (44,45).

Numerous approaches have been used to identify CpG islands that aredifferentially methylated in specific cell types, such as tumor-normalpairs for cancer-related methylation changes (46-48), or differentialparental origin for imprinted genes (49-50). However, there was only onereport of a systematic effort to identify CpG islands throughout thegenome that might be normally methylated (51) using a methyl-CPG bindingcolumn. However, the resulting sequences were mainly dispersed repeats,ribosomal DNA and other repeated sequences with no characterization ofunique, methylated CpG island.

There is a need in the art for identification of unique, methylated CpGislands so that imprinted genes can be identified.

SUMMARY OF THE INVENTION

One embodiment of the invention provides a method of forming embryonicgerm cells useful as a model system for studying imprinting. A male anda female mammal of the same species are mated to form a pregnant femalemammal. The male and the female mammals are sufficiently geneticallydivergent such that at least 50% of genes in resulting offspring have atleast one sequence difference between alleles of said genes. An embryois obtained from the pregnant female mammal at a stage of embryonicdevelopment between when 2-3 somites become visualizable and when gonadsare recognizable. The embryo is dissected and cells of the embryo aredissociated. The dissociated cells are cultured to provide embryonicgerm cell lines.

According to another embodiment of the invention a method is providedfor inducing imprinting in vitro. Mammalian embryonic germ cells arecultured in suspension culture under conditions in which the embryonicgerm cells differentiate. Expression of one or more imprintable geneschanges from approximately equal biallelic to preferentiallyuniparental.

One aspect of the invention provides a method of inducing imprinting invivo. One or more mammalian embryonic germ cells are injected into anude mouse. The embryonic germ cells differentiate and form ateratocarcinoma. Expression of one or more imprintable genes changesfrom approximately equal biallelic to preferentially uniparental.

Another aspect of the invention is a method of inducing imprinting invivo. A mammalian embryonic germ cell is injected into a blastocyst of amammal. The blastocyst is injected into a pseudopregnant mammal so thatthe blastocyst develops into a chimeric mammal. Expression of one ormore imprintable genes in somatic cells derived from the embryonic germcell becomes preferentially uniparental.

According to still another aspect of the invention an isolated andpurified mammalian embryonic germ cell line is provided. It expressesone or more imprintable genes in a biparental fashion. It forms cellswhich express one or more imprintable genes in a biparental manner. Itdifferentiates to form cells which express said one or more imprintablegenes in a preferentially uniparental fashion.

According to another embodiment of the invention a method of testingsubstances as candidate drugs is provided. An isolated and purifiedmammalian embryonic germ cell line as described above is contacted witha test substance. Imprinting of one or more imprintable genes isassayed.

Another embodiment of the invention provides a method of testingsubstances as candidates drugs. Isolated and purified mammalianembryonic germ cell line as described above are contacted with a testsubstance. Methylation of one or more imprintable genes is assayed.

According to still another aspect of the invention a method is providedfor making a chimeric animal which can be used as a model system forimprinting. A mammalian embryonic germ cell is transfected with a vectorwhich expresses a detectable marker protein. The embryonic germ cellexpresses one or more imprintable genes in a biparental manner. Thetransfected mammalian embryonic germ cells is injected into a blastocystof a mammal. The blastocyst is implanted into a pseudopregnant mammal.The blastocyst develops into a chimeric mammal. The chimeric mammalexpresses the one or more imprintable genes in a preferentiallyuniparental fashion. The present invention also provides chimericmammals made by the process.

Still another aspect of the invention provides a method for isolatingmethylated CpG islands. Eukaryotic genomic DNA is digested with a firstrestriction endonuclease which recognizes a recognition sequence foundin A/T rich regions of DNA or found in CpG island-poor regions of DNA.The eukaryotic genomic DNA is digested with a second restrictionendonuclease which recognizes a 4 base-pair sequence in unmethylated C/Grich regions. Fragments of at least 1 kb formed by the step of digestingare isolated and the fragments are inserted into bacterial vectors.Non-methylating, non-restricting bacteria are transformed with thebacterial vectors to propagate the vectors and render the fragments'progeny unmethylated. The unmethylated fragments are digested with athird restriction endonuclease which recognizes a sequence of at least 6base pair in G/C rich regions. The resulting fragments are isolated andinserted into bacterial vectors to form a library of sequences which areenriched for sequences derived from methylated CpG islands in theeukaryotic genome.

Also provided by the present invention are a library of fragments whichare enriched at least 100-fold in methylated CpG islands relative tototal genomic DNA.

Further aspects of the invention provide a method for testing substancesas candidate drugs. A nude mouse which has been injected with anembryonic germs cell to form a teratoma is contacted with a testsubstance. A test substance is identified as a candidate drug if itinhibits the growth of the teratoma or causes regression of theteratoma.

The present invention also provides a method of providing an assessmentof risk of developing cancer. Methylation status is determined in asample of a patient for a CpG island selected from the group identifiedin Table 2 (below). The methylation status of the CpG island is comparedto that found in a control group of healthy individuals. The patient isidentified as having an increased risk of developing cancer ifmethylation status of the CpG island is perturbed relative to themethylation status in the control group.

Another aspect of the invention is a method of providing diagnosticinformation relative to cancer. Methylation status of a CpG islandselected from the group identified in Table 2 is determined in a sampleof a tissue of a patient suspected of being neoplastic. The methylationstatus of the CpG island is compared to that found in a control sampleof said tissue which is apparently normal. The patient is identified ashaving an increased risk of developing cancer if methylation status ofthe CpG island is perturbed relative to the methylation status in thecontrol sample.

According to yet another aspect of the invention an isolated andpurified methylated CpG island is provided which is selected from thoseshown in Table 2.

Still another aspect of the invention provides a method of identifyingimprinted genes. A gene is identified which is within about 2 millionbase pairs of a CpG island identified in Table 2 in the human genome.One determines whether the gene is preferrentially uniparentallyexpressed. The gene is identified as an imprinted gene if it ispreferrentially uniparentally expressed.

According to another aspect of the invention an isolated and purifiedmethylated CpG island is provided. Surprisingly, the island ismethylated in both maternal and paternal alleles of a human.

Another aspect of the invention provides an isolated and purifiedmethylated CpG island which is biallelically methylated in some humansand not biallelically methylated in other humans. The methylated CpGisland thus comprises a methylation polymorphism.

The present invention thus provides the art with tools and methods foraccessing imprinted genes and using them for detecting birth defects,deiabetes, and cancers associated with aberrant imprinting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Experimental design. E8.5 F1 (129/SvEv×CAST/Ei) embryos weredissected near the base of the allantois to initiate PGC cultures fromwhich EG cell lines were established. EG cell lines were differentiatedin vitro by either of several methods, injected subcutaneously intoathymic nude mice to form teratocarcinoma, or transfected with a GFPvector and injected into the blastocysts of C57BL/6 to generate chimericmice, from which differentiated cells were purified by FACS.

FIG. 2A-2F. Characterization of mouse interspecific EG cell lines. (FIG.2A) Colony of EG cell line SJEG-1 cultured on a feeder layer of STOcells, viewed by phase contrast microscopy. (FIG. 2B) EG coloniesstained positive for alkaline phosphatase. (FIG. 2C) Embryoid bodiesformed upon spontaneous differentiation on plastic, viewed by phasecontrast microscopy. (FIG. 2D) A rhythmically contracting muscle bundleformed by differentiation of SJEG-1 cells transfected with αmMHCneovector. (FIG. 2E) Erythrocytes, epithelia, and (FIG. 2F) striatedmuscles in H&E sections of teratocarcinoma formed after injection ofSJEG-1 cells into nude mice. Scale bars: 10 μm in FIG. 2A, FIG. 2B, andFIG. 2D; 100 μm in FIG. 2C, FIG. 2E, and FIG. 2F.

FIGS. 3A and 3B. Partial imprinting establishment of EG cells induced byspontaneous in vitro differentiation on plastic. RNA and DNA wereprepared at varying times during differentiation. (FIG. 3A) SSCPanalysis of allele-specific expression of Kvlqt1, Igf2, and L23mrp.Paternal (Castaneus) and maternal (129) bands are indicated. The upperband is a nonspecific PCR product. (FIG. 3B) Changes in ratio ofparental allele expression of Kvlqt1, Igf2, H19, Snrpn, Igf2r, andL23mrp. Means and standard deviations are calculated from 4-7experiments each.

FIGS. 4A and 4B. Independence of imprinting establishment from method ofin vitro differentiation. (FIG. 4A) SNuPE analysis of allele-specificexpression of Snrpn. SJEG-1 cells were differentiated with all-transretinoic acid (RA), dimethyl sulfoxide (DMSO), and in methylcellulosemedium. Cells were harvested at 12 and 20 days of differentiation. (FIG.4B) SSCP analysis of allele-specific expression of Kvlqt1 inαMCneo-transfected SJEG-1 cells that were differentiated into cardiacmyocytes.

FIG. 5A-5E. Nearly complete imprinting of EG cells after in vivodifferentiation. (FIG. 5A) FACS analysis of SJEG-1 and SJEG-1/GFP18-1cell lines for GFP fluorescence intensity. SJEG-1/GFP18-1 was derivedfrom SJEG-1 by transfection with pEGFP-N3 vector and injected into theblastocyst of C57BL/6. (FIG. 5B) FACS analysis of spleen cells isolatedfrom a chimeric mouse and a non-chimeric littermate. Cells withfluorescence intensity greater than 40 units were collected, since thefluorescence intensity of >99.9% of cells derived from donor embryosfell below 30 units. (FIG. 5C, FIG. 5D, FIG. 5E) Analysis ofallele-specific expression of (FIG. 5C) Kvlqt1 and (FIG. 5D) Igf2 bySSCP, and (FIG. 5E) Snrpn by SNuPE, in GFP+ spleen cells obtained fromchimeric mice. Paternal (Castaneus) and maternal (129) bands areindicated. The upper constant band in (FIG. 5D) is a nonspecific PCRproduct.

FIGS. 6A and 6B. De novo establishment of allel-specific methylation ofH19 and Igf2 in EG cells by in vitro differentiation. (FIG. 6A) Analysisof H19 DMR Genomic DNA was digested with EcoR I (E), Msc I (M), and HpaII (H), and hybridized with a 450 bp probe, resulting in a 2.6 kb bandrepresenting methylated DNA, and a 1.74 kb band representingunmethylated DNA. The ratios of unmethylated to methylated bands were4.3, 2.3, 1.3, 1.2, and 0.83, at 0, 6, 10, 13, and 16 days,respectively. (FIG. 6B) Analysis of Igf2 DMR2. Genomic DNA was digestedwith BamH I (B) and Hpa II (h), and hybridized with a 640 bp proberesulting in a 2.45 kb band representing methylated DNA, and severallower molecular weight bands representing unmethylated DNA. An unrelatedcross-hybridizing band (C) variably appears as described previously(16). The ratios of methylated to unmethylated bands were 4, 4.8, 1.6,and 0.9, at 0, 10, 13, and 16 days, respectively.

FIG. 7A-7D. Nearly complete imprinting in differentiated human EG cells.(FIG. 7A) Monolayer culture of differentiated human EG cells (LV.EB)obtained from previously reported human EG cultures (21) under phasecontrast microscopy. Scale bar, 10 μm. (FIG. 7B) Nearly completemonoallelic expression of IGF2 in differentiated human EG cells. PCRproducts of genomic DNA were digested with Apa I revealingheterozygosity for A (236 bp) and B (173 bp) alleles. Digestion ofRT-PCR products (+RT) shows nearly complete preferential expression ofthe A allele, with no product in the absence of reverse transcriptase(−RT). (FIG. 7C) Complete monoallelic expression of H19 gene indifferentiated human EG cells. Digestion of PCR products with Alu Iresulted in both digested (128/100 bp doublet) and undigested (228 bp)alleles in genomic DNA, and only the undigested allele (148 bp) in cDNA.(FIG. 7D) Analysis of H19 DMR of differentiated human EG cells. GenomicDNA of differentiated EG cells (LV.EB) and a control tissue was digestedwith Sma I (H) and Pst I (P) and hybridized to a 1 kb probe, resultingin a 1.6 kb band representing methylated DNA, and a 1.0 kb bandrepresenting unmethylated DNA.

FIG. 8. Model of genomic imprinting in EG cells. For some imprintedgenes, EG cells derived from e8.5 embryos retain a gametic memory of theparental origin of the chromosome (colored boxes), althoughallele-specific silencing and methylation (black dots) are lost. Ondifferentiation into somatic cells, the EG cells re-establishallele-specific silencing and methylation. For EG cells derived fromolder embryos, this gametic memory has been erased, so that there is nochange in biallelic expression (green arrows) or DNA methylation ondifferentiation into somatic cells.

FIG. 9. Overall strategy for cloning methylated CpG islands. Malegenomic DNA from a Wilms tumor was digested with Hpa II and Mse I,fragments ≧1 kb in size were subcloned into a modified pGEM-4Z vectorand transformed into XL2-Blue MRF′, resulting in an expected 10×enrichment for methylated CpG islands, that was confirmed by Southernhybridization. Library DNA was then digested with Eag I, and fragmentsbetween 100 bp and 1500 bp were subcloned into pBC and transformed intoXL1-Blue MRF′ resulting in an expected 800× enrichment for methylatedCpG islands. Black ellipse depicts a methylated CpG island, clearellipse depicts an unmethylated CpG island. In step 1, thick arrowheadsabove the line depict Mse I sites (TTAA) and below the line depictunmethylated Hpa II sites (CCGG). In step 2, thick arrowheads depict EagI sites (CGGCCG). Enrichment estimates were based on an in silicoanalysis of frequencies of Mse I, Hpa II, and other CpG-rich restrictionendonucleases including Eag I, in CpG islands vs. non CpG island DNA:Mse I fragments ≧1 kb in size included 77% of CpG islands and 8% ofnon-CpG island DNA (0.77/0.08=10× enrichment). In the second step, 43%of the set of CpG islands would have been cloned by Eag I and thus for atwo-step cloning using Mse I and Eag 1, the fraction of methylated CpGislands expected is 0.43×0.77=0.33. The expected 800× enrichment isderived from the expected fraction of CpG islands after an Eag I digest(0.028) divided by the initial estimated fraction of methylated CpGislands based on the only known normally methylated autosomal CpGislands, i.e. those associated with imprinted genes.

FIG. 10. Methylation of SVA retroposons. DNA was digested with Mse I(M), Mse I+Hpa II (MH), or Mse I+Msp I (MM), electrophoresed on a 1.5%agarose gel, transferred to a nylon membrane and hybridized to a probeunique to the SVA element, SVA-U. LI: liver, LU: lung; fKI: fetalkidney; fLIM: fetal limb; SP: sperm; PT: parthenogenetic tumor(dysgerminoma).

FIG. 11A-11C. Methylation of MCI-S in normal tissues. DNA from varioustissues was digested with Mse I (M), Mse I+Hpa II (MH), or Mse I+Msp I(MM), electrophoresed on a 1.5% agarose gel, transferred to a nylonmembrane and hybridized with MCI-S clones. FIG. 1 IA) MCI-S aremethylated in blood. FIG. 11B) MCI-S/1-19 is methylated in fetal andadult somatic tissues. FIG. 11C) MCI-S are methylated in uniparental andgermline tissues. fCNS: fetal central nervous system; flu: fetal kidney;fLU: fetal lung; fSK: fetal skin; BR: brain; CO: colon; KI: kidney; LI:liver, OT: ovarian teratoma; CHM: complete hydatidiform mole.

FIG. 12A-12C. Methylation of MCI-D in normal tissues Tissue DNA wastreated as described in FIG. 3 and hybridized with MCI-D clones. FIG.12A) MCI-D are methylated in blood. FIG. 12B) MCI-D/2-78 is methylatedin fetal and adult somatic tissues. FIG. 12C) MCI-D methylation inuniparental and germline tissues: MCI-D are methylated in maternallyderived tissues and germline, unmethylated in sperm and completehydatidiform mole, and half-methylated in adult testis. fCNS: fetalcentral nervous system; fGU: fetal gut; fHE: fetal heart; fKI: fetalkidney; fLU: fetal lung; BR: brain; CO: colon; HE: heart; KI: kidney;LI: liver, OT: ovarian teratoma; CHM: complete hydatidiform mole; OV:ovary; fOV: fetal ovary; TE: testis; fTE: fetal testis.

FIG. 13. Variable methylation of MCI-T/2-d10 in normal tissue and Wilmstumor. DNA from normal blood, the tumor that was used to construct theMse I library (denoted WT*), and two pairs of matched Wilms tumor andnormal kidney from the same patients, was treated as described in FIG.11 and hybridized with MCI-T/2-d10.

FIG. 14. Sequence of isolated CpG islands are shown which are notavailable in public databases.

DETAILED DESCRIPTION OF THE DRAWINGS

We have derived highly polymorphic pluripotent EG cell lines from aninterspecific mouse cross, and have shown that these cells lackallele-specific expression and methylation, but acquire these featuresafter in vitro and in vivo differentiation into somatic cell lineages.These results have three important implications. First, these EG celllines represent the first in vitro model system in which genomicimprinting can be followed dynamically and the two alleles can bedistinguished. This system significantly enhances the identification andcharacterization of trans and cis-acting elements that modifyimprinting, and it also confers the advantages of extending suchinvestigations into an in vivo setting.

Second, these results demonstrate that gametic allele memory andallele-specific methylation are separable mechanisms. Our data suggest amodel in which undifferentiated EG cells obtained from e8.5 embryosretain a memory of their own parental origin even in the absence ofallele-specific silencing and methylation (FIG. 8). On differentiationinto somatic cell lineages, this gametic memory becomes manifest (FIG.8), as imprinted genes acquired allele-specific expression andmethylation. In EG cells derived from later stage embryos, this gameticmemory is lost (the PGCs from which the EG cells are derived wouldeventually become reprogrammed according to their own gender), and thuslate stage EG cells or PGCs are unable to undergo allele-specificsilencing and methylation on differentiation (18). Even in our earlystage EG cells, this gametic memory was not preserved for all imprintedgenes, as Igf2r was unable to attain imprinting after differentiation.This idea is also consistent with the observation that pre-implantationembryos may not show monoallelic expression of all imprinted genes (24).

This model also has important implications for understanding loss ofimprinting (LOI) in cancer (2). We have found that the normal pattern ofallele-specific methylation can be restored to at least some tumor cellswith loss-of-imprinting (LOI), suggesting that some gametic memory isretained in these cells (25). Similarly, Mitsuya et al. have found thathuman chromosomes introduced into mouse hybrids by microcell-mediatedtransfer can lose allele-specific expression but reacquire it after thecells are treated with differentiating agents (26). These observationsare consistent with our proposal that a gametic memory is distinct fromallele-specific expression and methylation at known DMRs, as we proposehere. While the molecular basis of this gametic memory is unknown,candidate mechanisms could include histone acetylation, specialchromatin structures, or DNA methylation elsewhere along the chromosome.

Third, since early EG cells did not for the most part lose a gameticimprinting mark, despite biallelic expression in those cells prior todifferentiation, we hypothesized that differentiated cell lineagesderived from early human EG cells would also show comparatively normalimprinting. This hypothesis was contrary to predictions (19) based onstudies of late mouse EG cells or PGCs (18). Our examination ofdifferentiated human EG-derived cells demonstrated normal imprinting atthe level of both gene expression and DNA methylation. Thus, genomicimprinting is unlikely to be a barrier to human embryonic stem celltransplantation.

We have also identified methylated CpG islands present in normal tissues(termed MCI). There have been systematic efforts to identify unique CpGislands differentially methylated in tumors (46-48) but no suchsuccessful efforts have been described for normally methylated CpGislands. While such sequences may have been suspected, this studyrepresents their first systematic identification in normal tissues, andas such represents a first step toward defining a “methylome”, i.e. thedistribution of methylation patterns layered on the distribution ofgenes in the genome.

MCI sequences appear to fall within distinct biological subgroups. Wedivided the MCI sequences into four categories, based on their copynumber and methylation pattern. The first group, MCI-R, is clearly themost abundant, and comprises high copy number sequences such as the SVAelement, and the intergenic and internal spacer sequences of ribosomalgenes. Methylation of one of these sequences, the rDNA nontranscribedspacer, was previously found after genomic purification from amethyl-CpG binding protein column (51), and one wonders whether thelarge number of these sequences obscured the identification of uniqueMCI's. The methylation of high copy number MCI sequences is notsurprising, as it is consistent with the hypothesis of that CpGmethylation arose as a host defense mechanism (63). This is particularlytrue of the SVA element, which is a high copy number retroposon.

Of greater interest in this study are the unique CpG islands methylatedin normal tissues. There has been great interest in CpG island sequencesbecause of their presumed function in regulation of expression ofhousekeeping genes (40), their potential involvement in silencing genesin tumors (44,45), and their role in providing a parentalorigin-specific mark to imprinted genes (42). Our prediction that 1-2%of CpG islands are methylated in normal tissues will likely alter ourperspective on CpG islands in general. An important direction of futureeffort will be to add to the number of known methylated CpG islands.There are several alternative approaches for generating additionalsecond libraries from the Mse I library, although the simplest approachfor identifying additional MCIs may be high throughput sequencing of theMse I library itself. We estimate that the Mse I library containsapproximately 77% of the MCI sequences, and we believe that all of theCpG islands within the Mse I library represent such sequences.

We were surprised by the large number of unique methylated CpG islandswe were able to identify using a restriction endonuclease-based cloningstrategy that eliminated most of the MCI-R sequences from the library.The two largest classes of these unique methylated CpG islands, MCI-Sand MCI-D, appear to have different properties, suggesting that they mayserve distinct potential functional roles. Specifically, the MCI-Ssequences were localized to high isochore regions near the ends ofchromosomes, and the MCI-D sequences generally showed a more centromericlocalization within low isochore regions. It is remarkable that theMCI-S, which are ubiquitously methylated, even in sperm, retain theirhigh CpG content, which also suggests that they may serve an importantrole. That role, however, would not appear to be gene silencing, sincemost of the MCI-S were within the body of transcriptionally activegenes.

The MCI-D sequences are particularly interesting for further study,because of their apparent differential methylation in the germline. Inparticular, these sequences may mark imprinted gene regions, as at leasttwo of these sequences in the Eag I library were found within imprintedgenes, namely IGF2R and HYMAI. Furthermore, most imprinted genes appearto lie within low isochore regions (PLAGL1, IGF2R, PEG1/MEST, SNRPN,PEG3, GNAS, unpublished data), like the MCI-D sequences. An intriguingpossibility is that a subset of low isochore domains, marked with MCI-Dsequences, harbor such genes.

Also surprisingly, most of these unique sequences were nottumor-specific (MCI-T) but were also methylated in normal tissues. Wesuspect that the MCI-T may represent a comparatively small fraction ofthe total number of unique methylated CpG islands. One possibility thatwill be the subject of further study is that the MCI-T may includesequences that are variably methylated in the population, such asMCI-T/2-d10. This is an intriguing idea because it suggests that themethylome might contribute to polymorphic variation in the population,which is consistent with the idea that methylation mutations may be morecommon in outbred populations than in laboratory strains (64).

Imprinting as used herein is the preferential expression of a specificparental allele, maternal or paternal. Typically it is associated withthe modification of a specific parental allele, such as by DNAmethylation, histone acetylation, histone phosphorylation, or histonemethylation. Imprinting can be assessed using any method known in theart for determining expression from a particular allele. Such techniquesinclude without limitation pyrosequencing for high throughput assaying,MALDI-TOF mass spectrometry, allele specific oligonucleotide DNAmicroarray, Hot-stop PCR (Uejima et al., Nat. Genet. 2000, 4: 375-6),SSCP (single stranded conformational polypmorphism assay), QS(quantitative sequencing), SNuPE (Single nucleotide primer extension),and allele-specific ligation assay. Unimprinted genes are typicallyexpressed in an approximately equal biallelic fashion, whereas imprintedgenes display preferential expression of a specific parental allele.Approximately equal biallelic expression may be as disparate as about40%:60%, preferably from about 45%: 55%, more preferably from about47.5%: 52.5%. Expression differences greater than this, such as 30%:70%,20%:80%, 10%:90%, and 5%: 95% are considered preferential expression ofa specific parental allele.

Methylated CpG islands which are repetitive (MCI-R) can be used asportable sites of genetic recombination, as indications of pastchromosomal rearrangements or as indications of past insertionelement-created mutations. Most CpG dinucleotides within a methylatedCpG island contain a methylated 5-position on the pyrimidine ring ofcytosine. The methylation level within a CpG island is believed to bequite hight, with at least 75%, 80%, 90%, 95%, or even 98% of thecytosine residues being methylated. Functionally, the methylated CpGislands survive the isolation procedure which involves restriction witha restriction endonuclease which cleaves at unmethylated CpGdinucleotides. Methylated CpG islands which are differentiallymethylated among maternal-derived and paternal-derived tissues (MCI-D)can be used as markers of the locations of imprinted genes. Typically,MCI-D are located within imprinted genes are adjacent to imprintedgenes. Adjacency is within 2×10⁶ base pairs, preferably within 1×10⁶base pairs, more preferably within 0.5×10⁶ base pairs. MCI-S and MCI-T,methylated CpG islands which are expressed similarly in uniparentaltissues and those which are differentially expressed in tumors andnormal tissues, can be used as methylation polymorphism markers in thepopulation. Thus they can be used as sequence polymorphisms,forensically, diagnostically, and predictively as risk factors fordisease traits.

Embryonic germ cells are useful as a dynamic model system for studyingimprinting. The ability to induce imprinting permits the analysis offactors which stimulate or inhibit the process. The factors can beendogenous or exogenously applied. It is desirable to use parentalanimals which are of the same species yet which are sufficientlygenetically divergent such that at least 50% of genes in resultingoffspring have at least one sequence difference between alleles of saidgenes. More preferably at least 60%, 70%, 75%, 80%, 90%, or 95% of thematernal and paternal genes in the offspring will be detectablydifferent. This greatly facilitates analysis of imprinting by renderingmost genes amenable to analysis of differential allelic expression.Suitable mammals which can be used include without limitation mice,rats, hamsters, guinea pigs, rabbits, goats, cows, sheep, pigs, horses,dogs, and cats.

Embryos are desirably removed from the pregnant female mammal at a stageof embryonic development between when 2-3 somites become visualizableand when gonads are recognizable. In mice, this stage is between day 7and 10 post conception. Obtaining embryos at such an early stage isbelieved to be beneficial in obtaining cells which have many genes whichare not yet imprinted. Embryos are dissected and cultured, preferably onfeeder cell layers. The posterior third of the emybryo can be dissectedand used to form dissociated cells. Alternatively, the genital ridge ofthe embryo is dissected out and used to form dissociated cells. Stillanother alternative method dissects out gonads of the embryo to formdissociated cells.

Once cell lines have been obtained they can be used for various assaysand tests. The cell lines express one or more imprintable genes in anapproximately equal biparental fashion, form cells which express one ormore imprintable genes in an approximately equal biparental manner, anddifferentiate to form cells which express said one or more imprintablegenes in a preferentially uniparental fashion The assays for imprintingcan be done in vitro or in vivo as is desired by the practicioner. Inone assay, the mammalian embryonic germ cells are grown in suspensionculture under conditions in which the embryonic germ cellsdifferentiate. The differentiated cells may or may not form an embryoidbody. Upon differentiation expression of one or more imprintable geneschanges from approximately equal biallelic to preferentiallyuniparental. Differentiation can be induced by growth on plastic in theabsence of feeder cells, by growth in the presence of dimethylsulfoxide,by growth in the presence of retinoic acid, by growth on amethyl-cellulose containing medium, or any other method known in theart. According to one particularly preferred method the germ cellscontain a selectable marker under transcriptional control of atissue-specific promoter, and the germ cells are subjected to selectionconditions to select for germ cells which have differentiated into alineage which activates the tissue-specific promoter.

A number of techniques are available for inducing and observingimprinting in vivo using the cell lines of the present invention. Themammalian embryonic germ cells can be injected into a nude mouse inwhich it will form a teratocarcinoma. One or more imprintable geneschange from approximately equal biallelic to preferentially uniparentalexpression upon formation of the teratocarcinoma. Another way to achieveimprinting in an in vivo model is to inject a mammalian embryonic germcell into a blastocyst of a mammal. The blastocyst is then implantedinto a pseudopregnant mammal so that the blastocyst develops into achimeric mammal, i.e., its somatic cells are not genetically identical.Expression of one or more imprintable genes in somatic cells derivedfrom the embryonic germ cell becomes preferentially uniparental. Thegerm cells used for formation of teratocarcinomas or chimericblastocysts can optionally be transfected with a vector which expressesa detectable marker protein. This makes distinguishing among the cellsof the mammal a simpler exercise.

Imprinting can be assayed directly in any of the models of the inventionby detecting parental allele specific expression. Alternatively, asurrogate for such expression can be used such as cytosine methylation,histone acetylation, histone phosphorylation, histone methylation.Methods for detecting such modifications are known in the art.

Test substances used to contact with the cell lines or chimeric mammalsof the present invention can be any natural, synthetic, or semisyntheticsubstance, whether a pure compound or a mixture of compounds. The testsubstances can be compounds or drug's which are known to have one mormore biological effects, or substances which are not known to have anybiological or physiological effects. If the test animal contains ateratoma, one can identify a test substance as a candidate drug if itinhibits the growth of the teratoma or causes regression of theteratoma. Techniques for assessing the growth of a teratoma orregression of a teratoma are well known in the art.

Methylated CpG islands can be isolated using a scheme as outlined inFIG. 9. Any restriction endonucleases can be used which have the desiredproperties specified. The properties are based on the frequency ofcleavage sites, and the preference of the cleavage sites for being inG/C or A/T rich regions. The CpG islands can be isolated from genomicDNA from males or females, from tumor or normal cells. Any type of tumoror normal tissue can be used as a source of cells. Once such methylatedCpG islands are isolated, they can be used for a number of differenttechniques. In one, they are tested to identify sequences which aredifferentially methylated between maternal and paternal chromosomes. Inanother technique they are tested to identify sequences which aredifferentially methylated between hydatidiform moles and teratomas. Inanother technique they are mapped to a genomic region. The CpG islandscan be used to identify an imprinted gene adjacent to the methylated CpGisland, as methylated CpG islands are markers for such genes. If a CpGisland is found to map to the same region as a disease which ispreferentially transmitted by one parent, an imprinted gene in theregion can be identified as a candidate gene involved in transmittingthe disease. The CpG islands can be used to screen populations ofindividuals for methylation. A sequence which is differentiallymethylated between individuals is a methylation polymorphism which canbe used to identify individuals.

Practice of the disclosed method for isolating CpG islands createslibraries which are enriched at least 100-fold, at least 250-fold, atleast 500-fold, or at least 750-fold in methylated CpG islands relativeto total genomic DNA. Preferably each library of fragements will containat least 25, at least 50, or at least 75 distinct members.

The particular CpG islands which have been found using the method of thepresent invention are disclosed in Table 2. These particular CpG islandscan be used to assess risk of developing cancer. Perturbed methylationof CpG islands relative to sequences in a control group of healthyindividuals suggests that the individual being tested are at increasedrisk of developing cancer. Any number of CpG islands can be tested insuch a method, but preferably at least 2, 5, 10, or 15 such islands willbe tested. An increased risk of developing cancer is determined if atleast 1 of 2, 3 of 5, 6 of 10, or 8 of 15 of the CpG islands haveperturbed methylation status relative to control group. Similarlyaberrant methylation of CpG islands can be determined where themethylation in a suspect tissue sample of a patient is compared to themethylation in an ostensibly healthy tissue sample of the patient.

CpG islands can be used to identify genes which are within about 2million base pairs of a CpG island identified in Table 2 in the humangenome. The genes are preferably within 1 million base pairs, and morepreferably within 500,000 base pairs. If the gene is preferrentiallyuniparentally expressed, then it is identified as an imprinted gene.

EXAMPLES Example 1

We used 129/SvEv mice as the mothers in the cross We chose CAST/Ei (Musmusculus castaneus) mice, separated from 129/SvEv by 5 million years inevolution, as the father in the cross, providing an average of onepolymorphic marker per 400 bp of transcribed sequence. The experimentalstrategy is summarized in FIG. 1, and it allows differentiation in vitroby a variety of mechanisms, including targeted differentiation using aselectable construct, and differentiation in vivo using chimeric mice.

Forty EG cell lines were derived from primordial germ cells (PGCs) of8.5 day embryos (4), as determined by colony morphology and positivealkaline phosphatase staining (FIG. 2A,B), and four of these lines werecharacterized in detail (termed SJEG-1, 2, 7, and 15). These EG celllines formed embryoid bodies after in vitro differentiation (FIG. 2C,D),teratocarcinomas in nude mice (FIG. 2E, F), and generated chimeric micewhen injected into the blastocyst of C57BL/6 mice (5). One male line wasalso used for subsequent germline transmission (5). Most of theimprinting studies were done on lines SJEG-1, 2, and 7.

Example 2

Partial establishment of imprinting in vitro. In order to distinguishthe two alleles of imprinted genes in these EG cell lines, we identifiedtranscribed polymorphisms distinguishing 129/SvEv and CAST/Ei in 5imprinted genes, Kvlqt1, Snrpn, Igf2, H19, and Igf2r, as well as thenonimprinted gene L23mrp as a negative control. For each gene, an assayfor allele-specific expression was then developed, as described inTable 1. TABLE 1 Transcribed polymorphisms and assay methods forallele-specific gene expression of EG cells derived from mouseinterspecific cross. Polymorphism Gene CAST/Ei¹ 129/SvEv Position² AssayMethod Kvlqt1 TCC C TGC TCC A TGC 1823 SSCP³ Igf2 GCA A TTC GCA G TTC777 SSCP³ H19 CTT G GAG CTT T GAG 1593 QS⁴ Snrpn CTA T AAT CTA C AAT 915SNuPE⁵ Igf2r ATC G ATG ATC A ATG 1549 SNuPE⁵ L23mrp ACC C GAG ACC T GAG407 SSCP³¹Polymorphisms were identified by direct sequencing of CAST/Ei genomicDNA. 129/SvEv sequence was identical to known Mus musculus musculussequence in GenBank, except that Kvlqt1 sequence was unavailable anddone here.²From first nucleotide of cDNA³Single strand conformation polymorphism (27).⁴Quantitative sequencing (28).⁵Single nucleotide primer extension (29).

Kvlqt1 shows preferential expression of the maternal allele throughoutdevelopment in this strain background (6). Prior to somaticdifferentiation of EG cells in vitro, Kvlqt1 showed approximately equalexpression of the two alleles (FIG. 3A). After differentiation byreplating on plastic in the absence of a feeder cell layer, Kvlqt1showed clear preferential expression of the maternal allele, whichincreased to a 6:1 ratio by day 16 (FIG. 3A), and this result was seenin all three cell lines tested (FIG. 3B). Like Kvlqt1, Igf2 showedapproximately equal biallelic expression of the two parental allelesprior to differentiation (FIG. 3A). However, after EG celldifferentiation, unlike Kvlqt1, which showed preferentialallele-specific expression in the same parental direction as F1offspring, Igf2 showed allele-specific expression but in oppositedirection to the F1 offspring. Thus, differentiated EG cells showedpreferential expression of the maternal allele of Igf2 (FIG. 3A). Whilethis was a surprising observation, it was consistent among differentcell lines (FIG. 3B). The expression of the maternal allele of IGF2 isalso consistent with an observation of allele reversal in embryonic stem(ES) cells (7). This may be a property of pluripotent embryonic stemcells (although note that in contrast to EG cells, imprinting showslittle or no change in ES cells (7)).

H19 normally shows reciprocal allele-specific expression to IGF2,perhaps due to competition for a shared enhancer (8). Consistent withthis pattern, H19 exhibited approximately equal expression of the twoparental alleles before differentiation, and preferential expression ofthe paternal allele after differentiation, changing from a ratio of 1:1to 3:1 after differentiation (FIG. 3B). Snrpn, which is preferentiallyexpressed from the paternal allele in somatic cells (9), also showedequal biallelic expression in undifferentiated EG cells (FIG. 3B). Afterdifferentiation, Snrpn showed preferential expression of the normallyexpressed paternal allele, at a ratio of 3:1 (FIG. 3B). In contrast,Igf2r showed approximately equal biallelic expression both before andafter differentiation, suggesting that for this gene, the gametic markhad been completely erased in EG cells (FIG. 3B).

As a negative control, we analyzed the nonimprinted gene L23mrp, whichis just outside of a contiguous imprinted gene domain that includesIgf2, H19, and Kvlqt1 (10). In contrast to Igf2, H19, and Kvlqt1, L23mrpshowed equal biallelic expression of the two parental alleles bothbefore and after in vitro differentiation (FIG. 3A,B). Furthermore, theratio of allele-specific expression of the imprinted genes afterdifferentiation differed significantly from that of L23mrp (p<0.01,two-tailed t-test). In summary, in vitro differentiation partiallyrestored imprinting to EG cells.

Example 3

Imprinting was independent of differentiation method. In order todetermine whether allele-specific expression in EG cells was caused bydifferentiation in vitro, or by the specific treatment used todifferentiate EG cells, we repeated these experiments by differentiatingthe cells in 3 other ways (4): differentiation in methylcellulosemedium; treatment with retinoic acid; and treatment with dimethylsulfoxide. In all cases, the results were identical to those seen onspontaneous differentiation on plastic in the absence of a feeder celllayer. For example, Snrpn showed equal biallelic expression of the twoparental alleles prior to differentiation, and preferential expressionof the paternal allele after differentiation in all cases, but withslight variation in the final ratio of parental alleles (FIG. 4A).

Embryoid bodies that result from in vitro differentiation of EG cellsshow considerable cellular heterogeneity, and not all of the cells aredifferentiated. In order to determine whether allele-specific expressionwould arise during differentiation down a specific cell lineage pathway,we used a genetic selection strategy to obtain lineage-specific EG celldifferentiation. We transfected EG cells with a vector containing theneo selectable marker gene under the control of a mouse α-cardiac myosinheavy chain gene promoter (11). Clones of transfected EG cells remainedundifferentiated, and showed equal biallelic expression of Kvlqt1, Igf2,H19, Snrpn, Igf2r and L23mrp (FIG. 4B and data not shown).Differentiation of transfected EG cells under G418 selection produced anetwork of rhythmically contracting myocyte bundles in culture (11)(FIG. 2D). Examination of these cells for allele-specific expressionshowed preferential allele expression similar to that seen using otherdifferentiation approaches, but with a slightly greater ratio ofallele-specific expression. For example, Kvlqt1 achieved a 9:1 ratio ofmaternal to paternal allele expression after cardiac myocyte-specificdifferentiation in vitro (FIG. 4B). Thus, establishment of imprintingwas due to differentiation itself, and not to the specific methods usedto induce it.

Example 4

Nearly complete imprinting establishment after differentiation of EGcells in vivo. To verify that the changes in imprinting we observed invitro also occurred during natural differentiation in vivo, we tookadvantage of the pluripotency of our EG cell lines to generate mousechimeras. In order to purify cells derived from these EG cells after invivo differentiation in chimeric mice, we first transfected EG cellswith a vector containing a modified GFP gene under the control of theCMV promoter (S) (FIG. 5A). We then injected the cells into C57B/J6blastocysts, which were introduced into pseudopregnant mice and allowedto develop to term (5). Spleens were removed from chimeras, and theEG-derived GFP(+) cells were purified by fluorescence-activated cellsorting (FACS) to 99% homogeneity (FIG. 5B). Purity of EG-derived cellsisolated from the chimeric mice was confirmed by measuring the alleleratio in genomic DNA for polymorphisms that distinguish the two strains(data not shown).

Analysis of imprinting of EG-derived cells isolated after in vivodifferentiation in chimeric mice indicated that all of the imprintedgenes studied showed the same pattern of allele-specific expressionfound after in vitro differentiation. However, after in vivodifferentiation, the degree of allele-specific expression was nearlycomplete. Thus, Kvlqt1 showed equal biallelic expression aftertransfection of the pEGFP-N3 vector and prior to blastocyst injection,and monoallelic expression of the maternal allele after in vivodifferentiation in three separate chimeric mice (FIG. 5C). Similarly,Igf2 showed monoallelic expression of the maternal allele in twoseparate chimeric mice and nearly monoallelic expression (>10:1) in athird (FIG. 5D). H19 also showed monoallelic expression of the paternalallele, the same allele preferentially expressed after in vitrodifferentiation (data not shown). Finally, Snrpn exhibited predominantexpression of the paternal allele (4:1 ratio) after in vivodifferentiation. As a control, L23mrp showed equal biallelic expressionafter in vivo differentiation (data not shown). Thus, in vivodifferentiation of EG cells caused nearly complete establishment ofimprint-specific expression.

Example 5

Establishment of differential DNA methylation during in vitrodifferentiation of EG cells. From all of the above experiments, it isclear that these EG cell chromosomes retain some memory of theirparental origin, but they do not manifest this memory as allele-specificexpression until the cells are differentiated. DNA methylation has beenshown previously to play a role in genomic imprinting, because micedeficient in DNA methyltransferase I show loss of imprinting (12). Inorder to determine whether DNA methylation represents the mechanism ofthe gametic mark, we analyzed the methylation status of two previouslywell-characterized differentially methylated regions (DMR).

Differential methylation in the H19 gene DMR, located −4 to −2 kbupstream of the transcriptional start site, is established in the gameteand stably maintained during early development (13). Our analysis ofundifferentiated EG cells revealed a hypomethylated pattern, at a ratioof 4.3:1 unmethylated to methylated bands (FIG. 6A). This result wasconsistent with the biallelic pattern of H19 expression inundifferentiated EG cells (FIG. 3B), since methylation of the H19 DMR isassociated with allele-specific silencing (14). However, with in vitrodifferentiation, H19 acquired a typical half-methylated pattern, similarto that seen in the parental and F1 mice, with a 1:1 ratio ofunmethylated to methylated bands (FIG. 6A). This change in methylationreflected well the change in expression from approximately biallelic topredominantly monoallelic in these cells after differentiation. Tofurther determine which parental allele of H19 became methylated afterin vitro differentiation, we analyzed the allele composition ofmethylated H19 DMR using a previously described method (13). Ouranalysis of differentiated EG cells revealed that the half-methylationpattern described above (FIG. 6A) was due to methylation of thenon-expressed allele (data not shown). Thus, the methylation wasallele-specific and related to silencing of the H19 gene duringdifferentiation.

Igf2 DMR2, within exon 6, is known to be the more closely linked DMR toIgf2 imprinting (15). We analyzed its methylation in EG cells by methodspreviously described (16). Analysis of undifferentiated EG cellsrevealed a hypermethylated pattern, at a ratio of 4:1 methylated tounmethylated bands (FIG. 6B), consistent with the biallelic expressionof Igf2 in undifferentiated cells (FIG. 3A,B), since the methylation ofIgf2 DMR2 is normally associated with the expressed allele (15). With invitro differentiation, Igf2 acquired a half-methylated pattern, with a1:1 ratio of methylated to unmethylated bands (FIG. 6B), consistent withthe predominantly monoallelic expression of Igf2 after differentiation(FIG. 3A,B). Thus, DNA methylation reflected the pattern of geneexpression of both Igf2 and H19, with a nonimprinted pattern of DNAmethylation before differentiation, and an imprinted pattern afterdifferentiation.

Example 6

Nearly complete imprinting in differentiated human EG cells. Pluripotenthuman EG cell cultures have recently been derived (17). The potentialtherapeutic use of these cells in medicine has received considerableattention, since they can be employed as an unlimited source for avariety of tissues used in human transplantation therapy. However, somerecent experiments using late mouse EG cells (e12.5) and PGCs(e14.5-16.5) suggested that genomic imprinting could not be established,and lack of imprinting is associated with developmental abnormalitiesand embryonic mortality (18). These results have raised widespreadpublic concern over the feasibility of human EG cells for therapeuticuse (19).

Because of these concerns, we endeavored to determine whether human EGcells can achieve genomic imprinting after differentiation, like mouseEG cells. We examined genomic imprinting in a differentiated monolayerculture of lineage-restricted cell types (20) (FIG. 7A), derived from ahuman EG culture reported previously (17). IGF2 was examined using anApa I polymorphism in exon 9 (21). While Apa I digestion revealed twoalleles in genomic DNA, analysis of cDNA showed a nearly completemonoallelic expression pattern (FIG. 7B), indicating a nearly completeestablishment of imprinting of IGF2 gene after in vitro differentiationof a human EG culture. H19 was then examined using an Alu I polymorphismin exon 5 (22). While Alu I digestion revealed two alleles in genomicDNA, analysis of cDNA showed a complete monoallelic expression pattern(FIG. 7C), indicating complete establishment of imprinting of H19 afterin vitro differentiation of human EG culture.

We further examined the methylation pattern of the H19 DMR (23) indifferentiated human EG cells. A double digestion of genomic DNA usingPst I and the methylation-sensitive enzyme Sma I revealed a 1.6 kbmethylated and a 1.0 kb unmethylated allele in control human tissuesamples (FIG. 7D). Analysis of differentiated EG-derived cells showedthe same methylation pattern seen in normal human tissues (FIG. 7D),indicating the establishment of a normal imprinting pattern in humanEG-derived cells.

Example 7

Experimental Design. We chose a restriction enzyme-based strategy forisolating methylated CpG islands over a PCR-based strategy, to avoidknown problems of amplification bias against GC-rich sequences, and inorder to obtain larger clone inserts than would be possible by aPCR-based approach. The source of DNA was a Wilms tumor from a male, toavoid cloning methylated CpG islands from the inactive X chromosome, andbecause this approach would identify either normally methylated CpGislands or those methylated specifically in tumors. The specific enzymeswere chosen by an in silico analysis of genomic sequences containing CpGislands. This analysis suggested a two-step approach (described indetail in FIG. 9). The first step involves digestion with Mse I and HpaII, followed by gel purification of fragments ≧1 kb in length. This stepwas predicted to enrich approximately 10-fold for CpG islands(enrichment was confirmed by a Southern blot, data not shown), whileeliminating all unmethylated CpG islands because of the methylcytosinesensitivity of Hpa II. This “Mse I library” was cloned into therestriction-negative strain XL2-Blue MRF′ to avoid bacterial digestionof methylated genomic DNA. CpG islands were further selected bydigesting Mse I library DNA with Eag I and subcloning, providing a totalexpected 800-fold enrichment for CpG islands in this “Eag I” library(see FIG. 9 brief description for details). Taking together theestimated library size and unique clones in it, with the predictedenrichment from the specific enzymatic strategy that was used, weestimated the total number of unique methylated CpG islands throughoutthe genome to be approximately 800, representing 1-2% of the totalnumber of CpG islands.

Construction of the Mse I library. DNA from a male Wilms' tumor samplewas isolated as described (52). 200 μg of DNA were digested overnightwith 1000 units of Hpa II (LTI) followed by a five hour digest with 600units of Mse I (NEB), according to the manufacturer's conditions, andthe volume was reduced using a SpeedVac concentrator (Savant). In orderto select for fragments ≧1 kb, the digest was passed through a sizeselection CHROMA-SPIN+TE-400 column (Clontech). Fragments between 1-9 kbwere purified from a 0.8% gel by electroelution and passed through anElutip-D column (S&S). The eluate was ethanol precipitated, cloned intothe compatible Nde I site of pGEM-4Z, which was first modified toabolish the Sma I site, transformed into the competent cells of therestriction-deficient strain XL2-Blue MRF′ (Stratagene), and plated ontoLB-Ampicillin agar plates. Library DNA was prepared directly from platesusing a plasmid Maxi kit (Qiagen).

Construction of the Eag I libraries. 100 μg of the Mse I library DNAwere digested with 1,000 u of Eag I (NEB) according to themanufacturer's conditions. The digest was ethanol precipitated, and 100to 1500 bp fragments were size-selected by purification from a 1.5%agarose gel, cloned into the Eag I site of pBC (Stratagene), andtransformed into XL1-Blue MRF′ (Stratagene). DNA from individualcolonies was prepared using a Perfect Prep kit (Eppendorf). In order toeliminate MCI-R sequences (Methylated CpG Island-Repetitive, seeresults) from the final Eag I library, 3.5 μg of the Mse I library waspurified, and half was digested with Acc I and half with Tth III1,pooled and digested with Dra III, Sal I, and Asc I, then re-transformedinto XL2-Blue MRF. This step eliminated >90% of the MCI-R sequences,while retaining approximately 30% of the MCI-S and MCI-D sequences(MCI-same in uniparental tissues, MCI-different in uniparental tissues,respectively, see results). Eag I libraries were prepared as describedabove, after gel purification from three overlapping fractions, 100-700bp, 400-1000 bp, 700-1500 bp, termed ES-1, 2, and 3, respectively.

DNA Sequencing. DNA sequencing was performed using an ABI 377 automatedsequencer following protocols recommended by the manufacturer(Perkin-Elmer). The sequences were analyzed by a BLAST search (53) ofthe NR, dbEST, dbGSS, dbHTGS, and dbSTS databases, and by GRAILanalysis. Chromosomal localization was performed by electronic PCR(ePCR, NCBI), or in some cases without matches using the GeneBridge 4radiation hybrids panel (Research Genetics).

Southern hybridization. Genomic DNA was digested with Mse I alone or MseI together with a methylcytosine-sensitive (Hpa II, LTI, or Sma I, NEB)or methyl-insensitive (Msp I or Xma I, NEB) restriction endonucleaseaccording to the manufacturer's conditions. Southern hybridization wasperformed as described (54).

Example 8

A class of high copy number methylated CpG islands. Our primary goal wasto identify unique methylated CpG islands throughout the genome.However, it quickly became apparent that most of the clones in the Eag Ilibrary represented high copy number methylated CpG islands. Themajority of these were derived from a sequence termed SVA, whichconstituted 70% of the Eag I library, and that was not previously knownto be methylated. The little-known SVA retroposon contains a GC-richVNTR region, which embodies a CpG island, between an Alu-derived regionand an LTR-derived region, only three such elements had previously beendescribed (55-57), although their methylation has not beencharacterized. We designed a probe, termed SVA-U, unique to the SVA andpresent in all of the SVA elements, to analyze copy number andmethylation of this sequence in genomic DNA. The copy number wasestimated to be 5000 per haploid genome (data not shown, L.S.-A. andA.P.F., in preparation). The SVA elements were found to be completelymethylated in all adult somatic tissues examined, including peripheralblood lymphocytes, kidney, adrenal, liver and lung, as well as fetaltissues including kidney, limb, and lung (FIG. 10). However, in germinaltissues SVA elements were hypomethylated but not completelyunmethylated. This methylation pattern was consistent with a retroposonmethylation pattern, where a group of active elements is unmethylated inthe germ line and maintains a high GC content, whereas in somatictissues the element is methylated and silenced. A somewhat less abundanthigh copy repeat, representing an additional 20% of the Eag I librarycorresponded to the nontranscribed intergenic spacer of ribosomal DNA,which was a known methylated repetitive sequence (58). A third high copymethylated sequence was the ribosomal DNA internal transcribed spacerand the 28S gene, comprising an estimated 5% of the Eag I library,suggesting that ribosomal gene methylation may be more extensive thanwas previously suspected. In summary approximately 25% of the Eag Ilibrary was accounted for by ribosomal DNA sequences, and 95% of the EagI library by ribosomal DNA and SVA together. For convenience, we termthis class of methylated CpG islands MCI-R (Methylated CpGIsland-Repetitive).

Example 9

Identification of Unique Methylated CpG Islands. One of the advantagesof our restriction enzyme-based two-step approach is that we could useit to eliminate the high copy number sequences described above. Towardthis end, we again performed an in silico analysis to identifycombinations of restriction endonucleases that could be used on the MseI library, to selectively eliminate the two common high copy numbermethylated CpG islands, and an Eag I library was re-constructedfollowing this procedure. This approach allowed us to uncover uniquemethylated CpG islands that might otherwise have been obscured.

After eliminating redundant clones, sixty-two unique clones werecharacterized in detail. All of the sequences were GC-rich, i.e. with ameasured (C+G)/N>50%, and they ranged in GC content from 55 to 79%.Forty-five (73%) of the clones showed an observed to expected CpGratio >0.6, meeting the formal definitional requirement of a CpG island.Thirty of these CpG islands were then characterized by detailed genomicanalysis, including radiation hybrid mapping of clones not within theknown database, and analysis of methylation in somatic and germlinetissues and in ovarian teratomas (OT) and complete hydatidiform moles(CHM), which are of uniparental maternal and paternal origin,respectively.

While the sequences recovered in this manner were predicted to bemethylated, we confirmed this assumption by direct examination ofgenomic DNA. Furthermore, as the original source of material was a Wilmstumor DNA sample, we had no a priori knowledge about the methylation ofthese sequences in normal tissue. Surprisingly, most were methylatednormally. More specifically, this analysis revealed that all of thesequences represented methylated CpG islands, and they could be dividedinto 3 major groups. The largest group consisted of sequences methylatedin all tissues examined, including fetal and adult somatic tissues,ovarian teratomas (OT), complete hydatidiform moles (CHM), and sperm.For example, clone 141 showed in blood an identical pattern after MseI+Hpa II digestion, as after Mse I digestion alone, compared to MseI+Msp I digestion which cut regardless of methylation (FIG. 1A). Thiswas true for other somatic tissues, as well as for ovarian teratoma,hydatidiform mole, and sperm (FIG. 11B,C). Altogether, half of theunique methylated CpG islands fell within this category, which we termMCI-S (Methylated CpG Island-Similar in uniparental tissues).

The second largest group, approximately 30% of the unique clones, weremethylated in normal somatic tissues, and unmethylated in completehydatidiform mole (CHM), which are uniparentally derived from the malegermline, as well as in sperm. For example, clone 2-78 showed anidentical pattern after Mse I+Hpa II digestion, as after Mse I digestionalone, in blood and other somatic tissues (FIG. 12A,B). However, clone2-78 showed complete digestion after Hpa II treatment of sperm andhydatidiform mole DNA, similar to the pattern seen after Msp I digestion(FIG. 12C). We termed this category MCI-D (Methylated CpGIsland-Different in uniparental tissues). All of the MCI-D sequenceswere methylated in OT and not CHM.

The final group, approximately 10% of the unique clones, wereunmethylated in normal tissue but methylated in tumors. For example,clone 2-d10 showed an identical methylation pattern in blood DNA afterMse I+Hpa II digestion as was seen after Mse I+Msp I digestion. However,Wilms tumor DNA, from which the Mse I library had been constructed, wasfully methylated (FIG. 13). Consistent with our nomenclature, thiscategory is termed MCI-T (Methylated CpG Island-Tumors). Though theMCI-T sequences were identified by virtue of their being methylated intumor tissue, they may represent sequences of polymorphic methylation inthe population, as a second individual showed methylation of 2-d10 inboth tumor and normal tissues and a third showed methylation in neithertumor nor normal tissues (FIG. 13).

Example 10

Chromosomal and isochore localization of unique methylated CpG islands.The remainder of the studies described here were performed on the twoclasses of unique CpG islands that are methylated in normal tissues,namely MCI-S and MCI-D. We first asked whether these sequences werefound in a unique location in the genome or were distributed moregenerally. Surprisingly, there was a striking difference in localizationwithin the genome of the MCI-S and MCI-D sequences. Virtually all of theMCI-S sequences were localized near the ends of chromosomes, either onthe last or the penultimate subband of the chromosome on which itresided (Table 2). In contrast, 70% of MCI-D sequences were localizedmore centromerically. This difference was highly statisticallysignificant (p<0.01, Fisher's exact test). The association of MCI-Ssequences near the ends of chromosomes is consistent with an observationof densely methylated GC-rich sequences near telomeres, although thatstudy did not describe methylated CpG islands (51). TABLE 2Characteristics of MCI-S and MCI-D Sequences. Name Accession GeneExpression Chromosome Isochore MCI-S/1-5 AL161774 — NA 13qtel H2 (54%)MCI-S/1-19 AF084481 WFS1 Br, Bra, Co, Ey, He, Ki, Li, 4p15 H2 (52%) Lu,Ly, Ov, Pa, Pl, Te, Ut MCI-S/1-30 AC008267 — NA 7q11-21 H1 (46%)MCI-S/1-41 NM_018104 FLJ10474 Br, Lu, Mu, Pr, Ut ND MCI-S 2-e3 AC010958— NA ND H1 (49%) MCI-S 2-h1 U60110 N-SGA-b Br, Bra, Lu, Pa, Pl, Pr, St,17q25 H1 (51%) Te, Ut MCI-S/3-110 AC023786 — NA ND H1 (51%) MCI-S/3-12AL157939 — NA 10q26 H2 (56%) MCI-S/3-20 AA001705 EST Retina NDMCI-S/3-c10 AK025954 FLJ22301 Br, Bra, Co, Ey, Ge, He, Ki, 1q44 Lu, Ly,Mu, Ov, Pa, Pl, Te MCI-S/4-f3 Hs.155647 EST Br, Co, Ma, Pr, Te 19p13 H3(66%) MCI-S/4-g6 AI361872 EST CGAP-CLL ND MCI-D/1-13 AP001403 — NA 18q23 L (43%) MCI-D/1-20 AL161645 — NA 10q26 H1 (48%) MCI-D/1-21 NM_016651LOC51339 infant/fetal brain 14q21  L (41%) MCI-D/2-4 U43342 NFATactivated T cells 20q13 H1 (48%) MCI-D/2-42 AC026454 — NA 16p11 H1 (49%)MCI-D/2-48 Hs.202088 EST CGAP-Lung 9p11-12  L (42%) MCI-D/2-78 AW090822EST Testis, CGAP-Brain 18q12  L (41%) MCI-D/2-e4 AC012191 — NA 8q21  L(39%) MCI-D 3-30 Hs.148365 5′ of EST fetal Lung/Testis/GCB 11q24 H1(45%) MCI-D 3-d4 AF241534 HYMAI fetal Heart, CGAP-CLL, Ge 6q24  L (39%)Expression data was derived from experimental data (not shown) as wellas from information in UniGene. Chromosome localization was derived fromePCR and radiation hybrids mapping; Isochore determination was accordingto the composition of the genomic sequence harboring the clone;Accession-GenBank accession; NA-not applicable, ND-not done, Br: brain,Bra: breast, Co: colon, Ey: eye, Ge: germ cell, He: heart, Ki: kidney,Li: liver, Lu: lung, Ly: lymph, Mu: muscle, Ov: ovary, Pa: parathyroid,Pl: placenta, Pr: prostate, St: stomach, Te: testis, To: tonsil, Ut:uterus, CGAP: Cancer Gene Anatomy Project, CLL: Chronic LymphocyticLeukemia, GCB: Germinal Center B-Cells.

We also questioned whether, in addition to their apparent chromosomalsegregation, the MCI-D and MCI-S sequences localized within compartmentsof differing genomic composition, i.e. isochores, which are regions ofseveral hundred kb of relatively homogeneous GC composition (59). Thisanalysis showed a striking segregation of MCI-D and MCI-S sequences.Approximately 75% of the MCI-S sequences fell within high isochoreregions (G+C≧50%), as might be expected from the high GC content ofmethylated CpG islands. Surprisingly, however, all of the MCI-Dsequences fell within low isochore regions (G+C<50%), i.e. of relativelylow GC content, despite the high GC content of the MCI-D sequencesthemselves (Table 1). This difference, like the chromosomal localizationwas also highly statistically significant (p<0.01, Fisher's exact test).Taken together, the comparison of MCI-S and MCI-D localization suggestthat they may lie within distinct chromosomal and/or isochorecompartments.

Example 11

Relationship of unique methylated CpG islands to genes. Most of theMCI-D and MCI-S sequences were localized within or near the codingsequence of known genes or of anonymous ESTs within the GenBankdatabase. These genes serve a wide variety of functions, including thewolframin gene, a transmembrane protein involved in congenital diabetes;sulphamidase, a lysosomal enzyme involved in Sanfilippo syndrome(MPS-IIIA); a cDNA similar to the gene for the extracellular matrixprotein tenascin; and an EST adjacent to the Peutz-Jeghers syndrome geneSTK11 (Table 2). Half of the MCI-S and one of the MCI-D sequencescorresponded to unique or very low copy number variable number tandemrepeat (VNTR) sequences. The location of the CpG islands within thesegenes appeared to differ between the MCI-S and MCI-D sequences, althoughthis difference was not statistically significant. Three of six MCI-Dsequences were localized within the promoter or contained the predictedtranscriptional start site. For example, MCI-D/2-78 matched ESTAWO90822, including the start of a 546 amino acid long ORF and apromoter predicted by GENSCAN just upstream of this sequence, andMCI-D/3-d4 was within the promoter and first exon of the HYMAI gene. Incontrast, none of 7 MCI-S sequences were found to include the start siteof transcription. For example, MCI-S/1-19 was within the last exon ofthe wolframin gene, and MCI-S/2-h1 was within the 5-6 exons of thesulphamidase gene. Finally, some of the MCI-D sequences may lie withinor near imprinted genes, consistent with their differential methylationin uniparental tissues. For example, the IGF2R gene, which contains anEag I site, was identified in the Eag I library (data not shown),consistent with the observation that one allele is methylated in normalcells. In addition, MSI-D/3-d4, which like other MSI-D sequences wasmethylated differentially in ovarian teratomas and hydatidiform moles,differed from most other MSI-D sequences in that it was only partiallymethylated in somatic tissues. Interestingly, this sequence was found tolie within the promoter and first exon of the HYMAI gene, which hasrecently also been demonstrated to be imprinted (60). Thus, a subset ofMCI-D sequences may mark the location of imprinted genes.

Example 12

Protocol for EG Cell Line Derivation

Media

1. STO Medium

DMEM supplemented with 10% FBS and Pen-Strep. Used for STO, Sl⁴-m220,Sl⁴-X9D3 culture.

2. EG Medium

DMEM with high glucose (4.5 g/liter) supplemented with 15% FBS(performance tested), non-essential amino acid (0.01 mM), L-glutamine (2mM), Pen-Strep, and 2-mercaptoethanol (0.1 mM).

Feeder Layer Preparation

1. Gelatin-Coated 24-Well Plate Preparation.

Add 0.1% gelatin in dH₂O into each wells and incubate for about onehour. Wash the well twice with PBS. Allow the well filled with PBS ordH₂O.

2. Prepare Feeder Layer.

1) STO Culture

STO cells are used as feeder layers for EG derivation and long termculture. Normally STO culture is maintained in 10 cm dish in STO mediaCulture must be split before reaching 85% confluence. Irradiationresistance of the maintained culture needs to be tested after a certainperiod of time. Should cells surviving irradiation found, throw away theculture and thaw a new vial of cells.

2) Prepare Feeder Layer

a. Trypsinize STO from culture the day before dissecting embryo. Suspendcells in culture media in 50 cc tubes. Irradiate cells for 4000 rads.Count the cells and pellet. Resuspend cells in media at 1.5×10⁵cells/ml. Add 1 ml (1.5×10⁵ cells) of cell suspension into each well ofgelatin-coated 24-well plate. Allow cells settle on the bottomovernight.

b. 2 hours before embryo dissection, change media in the wells into EGmedia supplemented with LIF (1000 U/ml), bFGF (1 ng/ml), and murine SCF(stem cell factor) (60 ng/ml).

Mice Mating

Natural mating is setup for 129/SvEv female and mus. Castanious male.Male must be older than 7 weeks and female must be between 8-18 weeks.

Put 2-3 females into a male cage in which only one male mouse is kept atthe end of the day. Check plug on females next morning. Separate pluggedfemales into new cages (one in each) and label the cage indicating themale partner.

Embryo Dissection

Dissect out the posterior third of the embryo from 8.5 dpc embryo.

Dissect out the genital ridge from 10.5 dpc embryo.

Dissect out the pair of gonads from 12.5 dpc embryo.

Primary Culture

1. Pool all dissected tissue fragments into a 15 cc tube. Rinse with PBSonce. Dissociate cells by adding 1 ml of 0.25% tyrosine/1 mM EDTAsolution and gently pipetting up and down for 2.5 min. Then add 5 ml ofEG media and keep pipetting up and down for about 2 min. Pellet cells at1000 rpm for 10 min. Resuspend cells into an appropriate volume (for 8.5dpc, 200 ul/embryo; 10.5 and 12.5 dpc, 1 ml/embryo) of EG mediasupplemented with LIF (1000 U/ml), bFGF (1 ng/ml), and murine SCF (stemcell factor) (60 ng/ml). Add 100 ul into each feeder layer coated wellsof 24 well plate.

2. Plate dissociated cell suspension into at least two separate plates.One with only a few wells plated for monitoring the survival andproliferation of PGCs in culture. Others with most or all of wellsplated for EG derivation.

3. After 6 days, some of the wells are stained for alkaline phosphataseeach day in order to assess the survival and growth of PGCs.

Secondary Culture and Line Cloning

1. At 9th days, prepare feeder layer plates.

2. After 10 days, cultures are trypsinized and replated: 2 hours beforetrypsinization, change media for feeder layer plate into EG medium. Washwells with PBS twice, and add 100 ul of 0.25% trypsin/1 mM EDTA intoeach well. Incubate plates at 37° C. for 2 min. Add 1 ml of EG mediainto each well and pipette up and down in the well. Collect trypsinizedcultures of all wells into a 15 cc tube, pellet cells and resuspendcells into appropriate volume (1 ml/well) of EG media supplemented withLIF (1000 U/ml). Add 1 ml into each well of prepared feeder layer plate.

3. Monitor the appearance of colonies in culture every day.

4. When most colonies expand into unaided visible sizes, trypsinize theculture with 0.05% trypsin/EDTA and isolate floating colonies form themedia. Isolated colonies are subjected to microdrop trepsinization(0.25% trypsin/EDTA) and plated into feeder layer of 24-well plates inEG media supplemented with LIF (1000 U/ml).

5. After two rounds of colony cloning, lines can be passed in 5 cmculture dish without further cloning.

Example 13

EG Cell Staining Protocol

Stage-Specific Mouse Embryonic Antigen-1 Staining

1. Culture EG cells on STO feeder layer on a chamber slide (Nunc).

2. Wash culture twice with PBS containing 2% calf serum and 0.1% sodiumazide.

3. Incubate culture with mouse monoclonal antibody (TG-1) againststage-specific mouse embryonic antigen-1 (at least 1:30 dilution) on icefor 30 min. (Ab from Dr. Peter Donovan in NCI).

4. After washed with PBS, culture are incubated for 30 min withFITC-conjugated Fab′ fragment of goat anti-mouse IgG (H+L) (Cappell, 1:5dilution) on ice.

5. Wash culture with PBS. Fix culture in 4% paraformaldehyde beforestaining for AP.

Alkaline Phosphatase Activity Staining

Use leukocyte alkaline phosphatase kit (catalog No. 85L-3R) from SIGMAand follow the accompanying protocol.

Example 14

Differentiation Essay for EG Cells

In Vitro Differentiation

Protocol I (Natural Differentiation)

1. EG culture on feeder layer is trypsinized (0.05% trypsin EDTA)lightly and pipetted gently to generate small clumps of cells. Separatethe EG cells from the irradiated STO cells as written below.

2. Transfer cell clumps into bacteriological plastic dishes and allowcell clumps to grow in suspension for 5 to 7 days. Most of clumpsdifferentiate into simple embryoid bodies, with a single outer layer ofextraembryonic ectoderm cells.

3. Return embryoid bodies back to tissue culture plastic dishes.Embryoid bodies will attach and give rise to a variety of cell typesover two weeks.

Separate EG Cells from STO Feeder Layer Cells

For all the following protocols, EG cultures are trypsinized (0.25%trypsin/EDTA) and single cell suspension is created. Plate cells into 10cm tissue culture dish at 37° C. for 1.5 hr to allow feeder layer cellsattach the bottom. Replate the media into another plate for anadditional 1.5 hr. Then collect media and pellet cells.

Protocol II (DMSO Induced Differentiation as Aggregates)

1. Resuspend cells into RA differentiation medium (DMEM supplementedwith 1% dimethyl sulfphoxide (DMSO), 10% FBS, L-Glutamine,Peniciline-Streptomycin) and transfer into bacterialogical dishes.

2. After 4 days, transfer cell aggregates into tissue culture dishes andculture with regular medium.

Protocol III (RA Induced Differentiation as Aggregates)

1. Resuspend cells into RA differentiation medium (DMEM supplementedwith 0.3 μM all-trans retinoic acid, 10% FBS, L-Glutamine,Peniciline-Streptomycin) and transfer into bacterialogical dishes.

2. After 4 days, transfer cell aggregates into tissue culture dishes andculture with regular medium.

Protocol IV (Differentiation in Methylcellulaose Medium)

1. Count EG cells and resuspend EG cells in methylcellulose medium* at aconcentration of 3.5×10⁵ cells/ml. Transfer 10 ml into each 10 cmbacteriological dish.

2. At day 4, split each dish into 2 dishes and grow for another 10 dayswith medium replaced daily.

* Methylcellulose medium (500 ml): Weight 3.7 g of NaHCO₃ and mix with10 g of BRL DMEM salt (pack for 1 liter media). Dissolv salts into 86 mlwater and pH to 6.9. Mix 20 ml of concentrated salt solution with 268 mlof DMEM, 50 ml FBS, 5 ml each of non-essential a.a., 2.3 ml ofL-glutamine, 5 ml of pen-strep. at 100× concentrations, and 4.1 ul of100% 2-mercaptoethanol. Filter the solution through 0.2 microm filtre.Add 150 ml of 2.2% (w/v) aqueous methylcellulose (Sigma, viscocity of 2%aqueous solution equal to 400 centipoises), mix and store at 4° for 1 hrbefore use.

Preparation of 2.2% aqueous methylcellulaose: Add 11 g ofmethylcellulaose power into bottle and add water to 500 ml. Stir thesolution in cold room overnight. Put bottle in microwave and boil thesolution three times (be careful not to spill the content). Tighten thecap right after the last boiling and leave the bottle in cold roomovernight. Store in refregirator.

Protocol V (DMSO Induced Differentiation as Single Cell Culture)

1. Resuspend cells into EG medium at a concentration of 3×10⁴ cells/ml,and plate into gelatinized tissue culture dishes. Culture for two daysallowing cells attach and grow.

2. Change to RA differentiation medium (DMEM supplemented with 1%dimethyl sulphoxide (DMSO), 10% FBS, L-Glutamine, non-essential a.a.,Peniciline-Streptomycin) and rteplace daily.

3. After 2 days, change to standard medium and replace daily.

Protocol VI (RA Induced Differentiation as Single Cell Culture)

1. Resuspend cells into EG medium at a concentration of 3×10⁴ cells/ml,and plate into gelatinized tissue culture dishes.

2. After two days, change to RA differentiation medium (DMEMsupplemented with 0.3 μM all-trans retinoic acid, 10% FBS, L-Glutamine,Peniciline-Streptomycin) and replace daily.

2. After 2 days, change to standard medium and replace daily.

In Vivo Differentiation

1. Harvest EG culture and wash three times with PBS.

2. Count cells and pellet/resuspend them into a concentration of 2×10⁶cells/ml in PBS.

3. Inject 1 ml cells subcutaneously into nude mice, three mice per cellline.

4. After 3-4 weeks, dissect out tumor and washed with PBS twice. Cuttumor into 2-3 pieces and fix in 4% neutral Formalin more than 1 day.Fixed tissue blocks are processed for histology. Sections are stainedwith hematoxylin and eosin.

REFERENCES

-   1. R. D. Nicholls, S. Saitob S, B. M. Horsthemke, Trends Genet. 14,    194-200 (1998); A. P. Feinberg, L. M. Kalikin, L. A. Johnson, J. S.    Thompson, Cold Spring Harb. Symp. Quant. Biol. 59,357-364 (1994).-   2. A. P. Feinberg, in Genomic Imprinting: Frontiers in Molecular    Biology, W. Reik and A. Surani, Eds. (Oxford University Press,    Oxford, 1998), chap. 9.-   3. Y. Kato et al., Develop. 126, 1823-1832 (1999); S.    Steghaus-Kovac, Science 286, 31 (1999).-   4. Derivation, maintenance, and in vitro differentiation of EG cell    lines: 8.5 d.p.c. embryos, resulted from crosses between male    CAST/Ei (Jackson Lab, 7-8 week old) and female 129/SvEv (Taconic    Farms, 7-8 week old) mice, were dissected according to Buehr and    McLaren (31). To derive EG cell lines, we primarily followed    Resnick, J. L. et al. (32) and Matsui, Y. et al. (32) with minor    modifications: Primary cultures were carried out in EG culture    medium (DMEM with 4.5 g/L glucose, 15% FBS, 100 units/ml    penicillin-streptomycin, 2 mM L-glutamine, 0.01 mM non-essential    amino acids, and 0.1 mM β-mercaptoethanol) supplemented with    leukemia inhibitory factor (LIF, 1000 units/ml), basic fibroblast    growth factor (bFGF, 1 ng/ml) and murine stem cell factor (SCF, 60    ng/ml). Cultures were trypsinized after nine days and replated in EG    culture medium without bFGF and SCF supplementation. Colonies were    picked, and individual EG cell lines were propagated on irradiated    STO feeder layers in EG medium with LIF (1000 unit/ml). Spontaneous    differentiation of EG cells on plastic was performed according to    Matsui, Y. et al. (32). Differentiation using RA, DMSO and    methylcellulose medium was carried out as described (33).-   5. pEGFP-N3 vector (Clontech) was transfected into SJEG-1 cells by    electroporation (250 μF, 0.2 kV). Clones with stable integration,    such as SJEG-1/GFP18-1, were obtained by G418 selection (500 μg/ml).    8 to 12 cells were injected into C57BL/6 blastocysts. The injected    embryos were transferred to pseudopregnant CD-1/VAF female mice. A    total of 87 blastocysts were injected and 4 living male chimeras    were obtained. Chimeric mice were identified by the agouti coat    color. Chimera 1-1 was mated with 3 female CD-1 mice, resulting in    three separate litters of offspring, in which about ⅓ were derived    from germline transmitted SJEG-1/GFP18-1 cells.-   6. S. Jiang, M. A. Hemann, M. P. Lee, A. P. Feinberg, Genomics 53,    395-399 (1998).-   7. W. Dean et al., Develop. 125, 2273-2282 (1998)-   8. P. A. Leighton, R. S. Ingram, J. Eggenschwiler, A.    Efstratladis, S. M. Tilghman, Nature 375, 34-39 (1995); L.    Thorvaldsen, K. L. Duran, M. S. Bartolomei, Genes Dev. 12, 3693-3702    (1998).-   9. S. E. Leff et al., Nat. Genet. 2, 259-264 (1992)-   10. Zubair et al., Genomics 45, 290-296 (1997)-   11. αmMHCneo vector was kindly provided by Dr. Lauren Field (34).    SJEG-1 cells were transfected by electroporation (250 μF, 0.2 kV).    Stable transfected lines were obtained by hygromycin selection (200    μg/ml). Transfected EG cells were differentiated on plastic and then    on tissue culture surfaces. Upon the appearance of spontaneously    contracting cells, G418 (400 μg/ml) was added until the culture    fully comprised rhythmically contracting muscle bundles.-   12. E. Li, C. Beard, R. Jaenisch, Nature 366, 362-365 (1993)-   13. K. D. Tremblay, J. R. Saam, R. S. Ingram, S. M. Tilghman, M. S.    Bartolomei, Nat. Genet. 9, 407-413 (1995).-   14. Brandeis et al., EMBO J. 12, 3669-3677 (1993); S.    Bartolomei, A. L. Webber, M. E. Brunkow, S. M. Tilghman, Genes Dev.    7, 1663-1673 (1993).-   15. R. Feil, J. Walter, N. D. Allen, W. Reik, Develop. 120,    2933-2943 (1994).-   16. T. Forne et al., Proc. Natl. Acad. Sci. USA 94, 10243-10248    (1997).-   17. M. J. Shamblott et al., Proc. Natl. Acad. Sci. USA 95,    13726-13731 (1998).-   18. T. Tada et al., Dev. Genes Evol. 207, 551-561 (1998); Y. Kato et    al., Develop. 126, 1823-1832 (1999).-   19. S. Steghaus-Kovac, Science 286, 31 (1999).-   20. A pluripotent human stem cell culture was derived from    primordial germ cells obtained from the gonadal ridges and attached    mesenteries of a 7-week post fertilization female embryo as    described (17). Embryoid bodies that formed spontaneously in the    presence of LIF were harvested then disaggregated by incubation in 1    mg/ml collagenase/dispase (Boehringer Mannheim) at 37° C. for 30    min. Monolayer cell cultures derived from these embryoid bodies were    routinely grown in RPMI 1640 and passaged weekly by using 0.05%    trypsin/0.53 mM EDTA.-   21. Analysis of IGF2 polymorphism and allele-specific expression was    performed essentially as described (30). PCR was performed using    [³²P]-ATP end-labeled primer, and the products were resolved on 5%    denaturing polyacrylamide gels following Apa I digestion.-   22. K. Hashimoto et al., Nat. Genet. 9, 109-110 (1995).-   23. W. Reik et al., Hun. Mol. Genet. 3, 1297-1301 (1995).-   24. K. Latham, Curr. Topics Dev. Biol. 43, 149 (1999); P. E.    Szabo, J. R. Mann, Genes Dev. 9, 3097-3108 (1995).-   25. J. M. Barletta, S. Rainier, A. P. Feinberg, Cancer Res. 57,    48-50 (1997).-   26. K. Mitsuya et al., Genes to Cells 3, 245-255 (1998). SSCP assays    were developed for each gene: Kvlqt1: PCR was performed using primer    set mLQT1-108/208 crossing multiple introns. 2 μl of the PCR    products was used for subsequent SSCP carried out in a 20-μl volume    containing 1×PCR buffer (BRL), 1 mM MgCl₂, 0.2 mM DNTP, 0.5 mM    unlabeled primer, 0.1 mM end-labeled primer, and 0.5 units of Taq    polymerase. Primer set mLQT1-U/L2 spanning two introns was used for    SSCP in which mLQT1-U was end-labeled with [³²P]-ATP. Reaction    products were electrophoresed on 8% SSCP gels (8% bis-acrylamide, 5%    glycerol, 0.25×TBE buffer, 4° C.) at 40 W for 6 hr. Igf2: PCR was    performed using primer set Igf2-U/L spanning an intron. 10 ng of    gel-purified PCR product was used as the template for subsequent    SSCP reactions conducted as described for Kvlqt1. Reaction products    were electrophoresed on 5% SSCP gels at 6 watts for 10 hr. L23mrp:    PCR was performed using primer set L23mrp-101/201 spanning an    intron. SSCP were performed using primer pair L23mrp-102/201 with 2    μCi of [α-³²P]-dATP added to each reaction. SSCP gels were run in    the same manner as for Kvlqt1. Sequences of primers used were as    follows: mLQT1-108, 5′-CCA CCA TCA AGG TCA TCA GGC GCA TGC-3′ (SEQ    ID NO: 1); mLQT1-208, 5′-GAG CTC CTT CAG GAA CCC TCA TCA GGG-3′ (SEQ    ID NO:2); mLQT1-U, 5′-TTT GTT CAT CCC CAT CTC AG-3′ (SEQ ID NO:3);    mLQT1-L2,5′-TTG TTC GAT GGT GGG CAG G-3′ (SEQ ID NO: 4); Igf2-U,    5′-GAC GTG TCT ACC TCT CAG GCC GTA CTT-3′ (SEQ ID NO:5); Igf2-L,    5′-GGG TGT CAA TTG GGT TGT TTA GAG CCA-3′ (SEQ ID NO: 6); Igf2-U1,    5′-GAT CTC TCT GCT CCA CTT CC-3′ (SEQ ID NO: 7); Igf2-L1, 5′-TTG TTT    AGA GCC AAT CAA AT-3′ (SEQ ID NO: 8); Igf2r-U, 5′-CTG GAG GTG ATG    AGT GTA GCT CTG GC-3′ (SEQ ID NO: 9); Igf2r-L, 5′-GAG TGA CGA GCC    AAC ACA GAC AGG TC-3′ (SEQ ID NO:10); Igf2r-2,5′-CTC CTC TGC GGG GCC    ATC-3′ (SEQ ID NO: 11); H19-U, 5′-CCA CTA CAC TAC CTG CCT CAG AAT    CTG C-3′ (SEQ ID NO: 12); H19-L2,5′-GGA ACT GCT TCC AGA CTA GG-3′    (SEQ ID NO: 13); H19-L1, 5′-ACG GAG ATG GAC GAC AGG TG-3′ (SEQ ID    NO: 14); Snrpn-U, 5′-TGC TGC TGT TGC TGC TAC TG-3′ (SEQ ID NO: 15);    Snrpn-L, 5′-GCA GTA AGA GGG GTC AAA AGC-3′ (SEQ ID NO: 16);    Snrpn-12, 5′-GCA GGT ACA CAA TTT CAC AAG AAG CAT T-3′ (SEQ ID    NO:17).-   27. Quantitative sequencing assay: PCR was performed with primer set    H19-U/L2 crossing an intron. Gel-purified PCR products were used in    the subsequent sequencing reaction with primer H19-L1. Two methods    of sequencing were used and shown to be concordant: (1)    fluorescence-based automatic sequencing; (2) cycle sequencing    reactions using the AmpliCycle sequencing kit and the provided    protocol (Perkin Elmer). Reaction products were run on 7% sequencing    gels at 90 W for 80 min and quantified on a PhosphorImager, with    genomic DNA as a control for allele intensity.-   28. SNuPE assays: Single nucleotide primer extension was performed    as described (35) with minor modifications. Snrpn: PCR was performed    with primer set Snrpn-U/L crossing an intron. SNuPE were performed    using primer Snrpn-12, and reaction products were resolved on 15%    denaturing polyacrylamide gels. Igf2r: PCR was performed with primer    set Igf2r-U/L crossing an intron. SNuPE was performed using primer    Igf2r-I2 as described above.-   29. S. Rainier, C. J. Dobry, A. P. Feinberg, Hum. Mol. Genet. 3,    386-386 (1994).-   30. M. Buehr and A. McLaren, in Guide to Techniques in Mouse    Development, P. M. Wassarman and M. L. DePamphilis, Eds. (Academic    Press, Inc., San Diego, 1993), vol. 225, chap. 4.-   31. J. L. Resnick, L. S. Bixier, L. Cheng, P. J. Donovan, Nature    359, 550-551 (1992); Y. Matsui, K. Zsebo, B. L. M. Hogan, Cell 70,    841-847 (1992).-   32. P. Szabo and J. R. Mann, Develop. 120, 1651-1660 (1994); N. D.    Allen, S. C. Barton, K. Hilton, M. L. Norris, M. A. Surani, Develop.    120, 1473-1482 (1994).-   33. M. G. Klug, M. H. Soonpaa, G. Y. Koh, L. J. Field, J. Clin.    Invest. 98, 216-224 (1996).-   34. J. Singer-Sam, PCR Methods Appl. 3, S48-S50 (1994); J.    Singer-Sam and A. D. Riggs, in Guide to Techniques in Mouse    Development, P. M. Wassarman and M. L. DePamphilis, Eds. (Press,    Inc., San Academic Diego, 1993), vol. 225, chap. 20; E. Szabo    and J. R. Mann, Genes Dev. 9, 1857-1868 (1995).-   35. Bird, A. P. (1986) Nature 321, 209-213.-   36. Bird, A. P., Taggart, M., Frommer, M., Miller, O. J., &    Macleod, D. (1985) Cell 40, 91-99.-   37. Gardiner-Garden, M., & Frommer, M. (1987) J. Mol. Biol. 196,    261-282.-   38. Antequera, F., and Bird, A. P. (1993). Proc. Natl. Acad. Sci.    USA 90, 11995-11999.-   39. Larsen, F., Gundersen, G., Lopez, R., & Prydz, H. (1992)    Genomics 13, 1095-1107.-   40. Cross, S. H., & Bird, A. P. (1995) Curr. Opin. Genet. Dev. 5,    309-314.-   41. Yen, P. H., Patel, P., Chinault, A. C., Mohandas, T., &    Shapiro, L. (1984) Proc. Natl. Acad. Sci. USA 81, 1759-1763.-   42. Razin, A. & Cedar, H. (1994) Cell 77, 473-476.-   43. Barlow, D. P. (1995) Science 270, 1610-1613.-   44. Merlo, A., Herman, J. G., Mao, L., Lee, D., Gabrielson, E.,    Burger, P. C., Baylin, S. B., & Sidransky, D. (1995) Nat. Med 1,    686-692.-   45. Herman, J. G., Latif, F., Weng, Y., Lerman, M. L., Zbar, B.,    Liu, S., Samid, D., Duan, D. R., Gnarra, G. R., et al. (1994) Proc.    Natl. Acad. Sc. USA 91, 9700-9704.-   46. Toyota, M., Ho, C., Ahuja, N., Jair, K.-W., L1, Q., Ohe-Toyota,    M., Baylin, S. B., & Issa, J.-P. J. (1999) Cancer Res. 59,    2307-2312.-   47. Huang, T. H.-M., Perry, M. R., & Laux, D. E. (1999) Hum. Mol.    Genet. 8, 459-470.-   48. Shiraishi, M., Chuu, Y. H., & Sekiya, T. (1999) Proc. Natl.    Acad. Sci. USA 96, 2913-2918.-   49. Hayashizaki, Y., Shibata, H., Hirotsune S., Sugino, H., Okazaki,    Y., Sasaki, N., Hirose, K., Imoto, H., Okuizumi, H., et al. (1994)    Nat. Genet. 6, 33-40.-   50. Plass, C., Shibata, H., Kalcheva, I., Mullins, L., Kotelevtseva,    N., Mullins, J., Kato, R., Sasaki, H., Hirotsune, S., et al. (1996)    Nat. Genet. 14, 106-109.-   51. Brock, G. J. R., Charlton, J., & Bird, A. P. (1999) Gene 240,    269-277.-   52. Gross-Bellard, M., Oudet, P., & Chambon, P. (1973) Eur. J.    Biochem. 36, 32-38.-   53. Altschul, S. F., Gish, W., Miller, W., Myers, E. W., &    Lipman, D. J. (1990). J. Mol. Biol. 215,403-410.-   54. Dyson, N. J. (1991) in Essential Molecular Biology: A Practical    Approach, Vol 2, ed. Brown, T. A. (IRL Press, Oxford), pp. 111-156.-   55. Kawajiri, K., Watanabe, J., Gotoh, O., Tagashira, Y., Sogawa,    K., & Fujii-Kuriyama, Y. (1986) Eur. J. Biochem. 159, 219-225.-   56. Zhu, Z. B., Hsieh, S., Bently, D. R., Campbell, D. R., &    Volanakis, J. E. (1992) J. Exp. Med. 175, 1783-1787.-   57. Shen, L., Wu, L. C., Sanlioglu, S., Chen, R., Mendoza, A. R.,    Dangel, A. W., Carroll, M. C., Zipf, W. B., & Yu, C. Y. (1994) J.    Biol. Chem. 269, 8466-8476.-   58. Brock, G. J. R., & Bird, A. P. (1997) Hum. Mol. Genet. 6,    451-456.-   59. Bernardi, G. (1995) Ann. Rev. Genet. 29, 445-476.-   60. Arima, T., Drewell, R. A., Oshimura, M., Wake, N., &    Surani, A. (2000) Genomics 67,248-255.-   61. Brandeis, M., Frank, D., Keshet, I., Siegfried, Z., Mendelsohn,    M., Nemes, A., Temper, V., Razin, A. & Cedar, H. (1994) Nature 371,    435-438.-   62. Bird, A. P. (1980) Nucl. Acids. Res. 8, 1499-1504.-   63. Yoder, J. A., Walsh, C. P., & Bestor, T. H. (1997) Trends    Genetics 13, 335-340.-   64. Cubas, P., Vincent, C., & Coen, E. (1999) Nature 401, 157-16

1. A method of forming embryonic germ cells useful as a model system forstudying imprinting, comprising: mating a male and a female mammal ofthe same species to form a pregnant female mammal, wherein the male andthe female mammals are sufficiently genetically divergent such that atleast 50% of genes in resulting offspring have at least one sequencedifference between alleles of said genes; obtaining an embryo from thepregnant female mammal at a stage of embryonic development between when2-3 somites become visualizable and when gonads are recognizable;dissecting said embryo, dissociating cells of said embyro, and culturingthe dissociated cells to provide embryonic germ cell lines.
 2. Themethod of claim 1 wherein the mammals are mice.
 3. The method of claim 2wherein the embryo is obtained at day 7-10 post conception.
 4. Themethod of claim 1 wherein the female mammal is a 129/SvEv mouse.
 5. Themethod of claim 1 wherein the male mammal is a CAST/Ei mouse.
 6. Themethod of claim 1 wherein the dissociated cells are cultured on a feedercell layer.
 7. The method of claim 1 wherein the posterior third of theemybryo is dissected and used to form dissociated cells.
 8. The methodof claim 1 wherein the genital ridge of the embryo is dissected out andused to form dissociated cells.
 9. The method of claim 1 wherein gonadsof the embryo are dissected out and used to form dissociated cells. 10.The method of claim 1 wherein the wherein the male and the femalemammals are sufficiently genetically divergent such that at least 60% ofgenes in resulting offspring have at least one sequence differencebetween alleles of said genes.
 11. The method of claim 1 wherein thewherein the male and the female mammals are sufficiently geneticallydivergent such that at least 75% of genes in resulting offspring have atleast one sequence difference between alleles of said genes.
 12. Themethod of claim 1 wherein the wherein the male and the female mammalsare sufficiently genetically divergent such that at least 90% of geneshave at least one sequence difference between alleles of said genes 13.The method of claim 1 wherein the wherein the male and the femalemammals are sufficiently genetically divergent such that at least 95% ofgenes in resulting offspring have at least one sequence differencebetween alleles of said genes.
 14. A method of inducing imprinting invitro, comprising: culturing mammalian embryonic germ cells insuspension culture under conditions in which the embryonic germ cellsdifferentiate, whereby expression of one or more imprintable geneschanges from approximately equal biallelic to preferentiallyuniparental.
 15. The method of claim 14 wherein the germ cells are grownon plastic in the absence of feeder cells.
 16. The method of claim 14wherein the germ cells are grown in the presence of dimethylsulfoxide.17. The method of claim 14 wherein the germ cells are grown in thepresence of retinoic acid.
 18. The method of claim 14 wherein the germcells are grown on a methyl-cellulose containing medium.
 19. The methodof claim 14 wherein the germ cells contain a selectable marker undertranscriptional control of a tissue-specific promoter, and the germcells are subjected to selection conditions to select for germ cellswhich have differentiated into a lineage which activates thetissue-specific promoter.
 20. The method of claim 14 wherein the germcells form an embryoid body.
 21. A method of inducing imprinting invivo, comprising: injecting one or more mammalian embryonic germ cellsinto a nude mouse, whereby the embryonic germ cells differentiate andform a teratocarcinoma and whereby expression of one or more imprintablegenes changes from approximately equal biallelic to preferentiallyuniparental.
 22. A method of inducing imprinting in vivo, comprising:injecting a mammalian embryonic germ cell into a blastocyst of a mammal;implanting the blastocyst into a pseudopregnant mammal so that theblastocyst develops into a chimeric mammal, whereby expression of one ormore imprintable genes in somatic cells derived from the embryonic germcell becomes preferentially uniparental.
 23. The method of claim 22wherein the mammalian embryonic germ cell is transfected with a vectorwhich expresses a detectable marker protein, prior to the step ofinjecting.
 24. An isolated and purified mammalian embryonic germ cellline which: expresses one or more imprintable genes in a biparentalfashion; forms cells which express one or more imprintable genes in abiparental manner; differentiates to form cells which express said oneor more imprintable genes in a preferentially uniparental fashion. 25.The isolated and purified mammalian embryonic germ cell line of claim 24which is a mouse cell line.
 26. The isolated and purified mammalianembryonic germ cell line of claim 24 which differentiates in vitro. 27.The isolated and purified mammalian embryonic germ cell line of claim 24which differentiates in vivo.
 28. The isolated and purified mammalianembryonic germ cell line of claim 24 which imprints in vitro.
 29. Theisolated and purified mammalian embryonic germ cell line of claim 24which imprints in vivo.
 30. A method of testing substances as candidatedrugs comprising: contacting the isolated and purified mammalianembryonic germ cell line of claim 24 with a test substance; assayingimprinting of one or more imprintable genes.
 31. The method of claim 30further comprising the step of: identifying a test substance as acandidate drug for treating cancer if the test substance enhancesimprinting of a gene whose imprinting is lost in cancer, or if the testsubstance inhibits imprinting of a gene whose imprinting is gained incancer.
 32. The method of claim 30 wherein differentiation of themammalian embryonic germ cell line is induced before, after, or duringthe step of contacting.
 33. The method of claim 30 wherein the mammalianembryonic germ cell line is transfected with a vector encoding a markerprotein, the mammalian embryonic germ cell line is injected into ablastocyst, and the blastocyst is implanted in a pseudopregnant female.34. The method of claim 30 wherein the step of assaying is done bysingle strand conformation polymorphism analysis.
 35. The method ofclaim 30 wherein the step of assaying is done by quantitativesequencing.
 36. The method of claim 30 wherein the step of assaying isdone by single nucleotide primer extension.
 37. The method of claim 30wherein the step of assaying is done by hot stop PCR.
 38. A method oftesting substances as candidates drugs comprising: contacting theisolated and purified mammalian embryonic germ cell line of claim 24with a test substance; assaying methylation of one or more imprintablegenes.
 39. The method of claim 38 further comprising the step of:identifying a test substance as a candidate drug for treating cancer ifthe test substance enhances methylation of a gene whose methylation islost in cancer, or if the test substance inhibits methylation of a genewhose methylation is gained in cancer.
 40. A method of making a chimericanimal which can be used as a model system for imprinting, comprising:transfecting a mammalian embryonic germ cell with a vector whichexpresses a detectable marker protein, wherein the embryonic germ cellexpresses one or more imprintable genes in a biparental manner;injecting the transfected mammalian embryonic germ cells into ablastocyst of a mammal; implanting the blastocyst into a pseudopregnantmammal, whereby the blastocyst develops into a chimeric mammal, whereinthe chimeric mammal expresses the one or more imprintable genes in apreferentially uniparental fashion.
 41. A chimeric mammal made by theprocess of claim
 40. 42. The method of claim 30 whereinpost-translational modification of histones is determined.
 43. Themethod of claim 31 wherein post-translational modification of histonesis determined.
 44. The method of claim 32 wherein post-translationalmodification of histones is determined.
 45. The method of claim 33wherein post-translational modification of histones is determined.
 46. Amethod for isolating methylated CpG islands comprising the steps of: a.digesting eukaryotic genomic DNA with a first restriction endonucleasewhich recognizes a recognition sequence found in A/T rich regions of DNAor found in CpG island-poor regions of DNA; b. digesting the eukaryoticgenomic DNA with a second restriction endonuclease which recognizes a 4base-pair sequence in unmethylated C/G rich regions; c. isolatingfragments of at least 1 kb formed by the step of digesting and insertingthe fragments into bacterial vectors; d. transforming non-methylating,non-restricting bacteria with the bacterial vectors to propagate thevectors and render the fragments' progeny unmethylated; e. digesting theunmethylated fragments with a third restriction endonuclease whichrecognizes a sequence of at least 6 base pair in G/C rich regions; f.isolating the resulting fragments and inserting said fragments intobacterial vectors to form a library of sequences which are enriched forsequences derived from methylated CpG islands in the eukaryotic genome.47. The method of claim 46 further comprising the step of eliminatingundesired repetitive elements by digesting the resulting fragmentsreferred to in step (f) with a fourth restriction endonuclease whichrecognizes a unique site in the repetitive elements.
 48. The method ofclaim 46 wherein the first restriction endonuclease is Mse I.
 49. Themethod of claim 46 wherein the second restriction endonuclease is Hpa II50. The method of claim 46 wherein the third restriction endonuclease isEag I.
 51. The method of claim 46 wherein the fourth restrictionendonuclease recognizes a site in element SVA.
 52. The method of claim46 wherein the eukaryotic genomic DNA is isolated from a male.
 53. Themethod of claim 46 wherein the eukaryotic genomic DNA is isolated from atumor.
 54. The method of claim 46 wherein the eukaryotic genomic DNA isisolated from a Wilm's tumor.
 55. The method of claim 46 furthercomprising the step of: testing one or more members of the library ofsequences which are enriched for sequences derived from methylated CpGislands to identify sequences which are differentially methylatedbetween maternal and paternal chromosomes.
 56. The method of claim 46further comprising the step of: testing one or more members of thelibrary of sequences which are enriched for sequences derived frommethylated CpG islands to identify sequences which are differentiallymethylated between hydatidiform moles and teratomas.
 57. The method ofclaim 46 further comprising the step of: mapping one or more members ofthe library of sequences to a genomic region, whereby location of amethylated CpG island island is determined.
 58. The method of claim 57further comprising the step of: identifying an imprinted gene adjacentto the methylated CpG island; identifying a disease which ispreferentially transmitted by one parent and which is genetically linkedto region of genomic DNA which contains the imprinted gene, whereby theimprinted gene is thereby indicated as a candidate gene involved intransmitting the disease.
 59. The method of claim 46 further comprisingthe step of: testing a population of individuals for methylation of amember of the library of sequences, whereby a sequence which isdifferentially methylated between individuals is a methylationpolymorphism which can be used to identify individuals.
 60. A library offragments which are enriched at least 100-fold in methylated CpG islandsrelative to total genomic DNA.
 61. The library of fragments of claim 60which comprises at least 50 distinct members.
 62. A method for testingsubstances as candidate drugs, comprising: contacting a mouse made bythe process of claim 21 with a test substance; identifying a testsubstance as a candidate drug if it inhibits the growth of the teratomaor causes regression of the teratoma.
 63. A method of providing anassessment of risk of developing cancer, comprising the steps of:determining methylation status of a CpG island selected from the groupidentified in Table 2 in a sample of a patient; comparing themethylation status of the CpG island to that found in a control group ofhealthy individuals; identifying the patient as having an increased riskof developing cancer if methylation status of the CpG island isperturbed relative to the methylation status in the control group. 64.The method of claim 63 wherein the status of at least 5 CpG islands isdetermined and the patient is identified as having an increased risk ifat least 3 of said CpG islands have perturbed methylation statusrelative to control group.
 65. A method of providing diagnosticinformation relative to cancer, comprising the steps of: determiningmethylation status of a CpG island selected from the group identified inTable 2 in a sample of a tissue of a patient suspected of beingneoplastic; comparing the methylation status of the CpG island to thatfound in a control sample of said tissue which is apparently normal;identifying the patient as having an increased risk of developing cancerif methylation status of the CpG island is perturbed relative to themethylation status in the control sample.
 66. The method of claim 65wherein the status of at least 5 CpG islands is determined and thepatient is identified as having an increased risk if at least 3 of saidCpG islands have perturbed methylation status relative to controlsample.
 67. An isolated and purified methylated CpG island which isselected from those shown in Table
 2. 68. The CpG island of claim 67which retains its methylation pattern found in a human.
 69. The CpGisland of claim 68 wherein the methylation pattern found in a human ismethylated in normal indviduals, but not in diseased or disease-proneindividuals.
 70. The CpG island of claim 68 wherein the methylationpattern found in a human is unmethylated in normal indviduals, butmethylated in diseased or disease-prone individuals.
 71. The CpG islandof claim 68 wherein the methylation pattern found in a human ismethylated in normal tissues, but not in diseased or diseased tissues.72. The CpG island of claim 68 wherein the methylation pattern found ina human is unmethylated in normal tissues, but methylated in diseasedtissues.
 73. The CpG island of claim 67 which is devoid of itsmethylation pattern found in a human.
 74. A method of identifyingimprinted genes comprising the steps of: identifying a gene which iswithin about 2 million base pairs of a CpG island identified in Table 2in the human genome; determining whether the gene is preferrentiallyuniparentally expressed; identifying the gene as an imprinted gene if itis preferrentially uniparentally expressed.
 75. An isolated and purifiedmethylated CpG island which is methylated in both maternal and paternalalleles of a human.
 76. The isolated and purified methylated CpG islandof claim 75 wherein the human is healthy.
 77. The isolated and purifiedmethylated CpG island of claim 75 wherein the methylation is notassociated with a disease state.
 78. An isolated and purified methylatedCpG island which is biallelically methylated in some humans and notbiallelically methylated in other humans, thus comprising a methylationpolymorphism.
 79. The CpG island of claim 78 which is methylated innormal tissue of a human having a tumor but not in tumor tissue of thehuman.
 80. The CpG island of claim 78 which is methylated in both normaland tumor tissue of a human who has a tumor.